Scott Peterson has made more than 30 visits to Iran since the mid-1990s. But following the disputed election of 2009, which resulted in months of political unrest, few journalists have been able to report from inside the country. Yet changes are now under way, with the surprise election of centrist President Hassan Rouhani in June, and a new interim nuclear deal agreed on with six world powers in Geneva in November. This is a diary of Mr. Peterson's first seven days back in the Islamic Republic, after 4-1/2 years away.

CAB SERENADE

Touchdown at 3:36 a.m.

The Iranian passengers work their way through immigration, their passports stamped by young officers smartly dressed in pale green uniforms. The line for foreigners is shorter, and I am the only American. These days, a generation after Iran's 1979 Islamic Revolution, which turned flag burning and chants of "Death to America" into an art form, Iranians don't get to add many US passports to their database.

Yet despite the decades of mutual hostility, Iran still has perhaps the most pro-American population in the Middle East. It shows in the reaction of the officer, who slips me a quiet handshake and says, "Welcome."

A sign on a nearby cubicle reads "Finger Print Room," and I ask if this is still part of the routine. Iran began taking fingerprints of visiting US citizens in the late 1990s, in reaction to a US decision to do the same to all Iranians entering the United States. Back then the process was a messy affair, with an ink pad and tissues, leaving my fingers stained purple for days.

With a smile the officer reassures me that prints won't be necessary, and I raise my arms in mock surprise and gratitude. He laughs but is immediately overruled by a superior, who delicately leads me to a cubicle, apologizing: "So sorry, so sorry. It will just take a moment."

And it does, using a digital scanner. I'm the last to leave the airport – waved through without the usual X-ray of my bag by officials sympathetic of the late hour. All the proceedings in the cavernous arrival hall are watched over by large portraits of the Islamic Republic's only two supreme leaders, both black-turbaned ayatollahs: Ruhollah Khomeini and Ali Khamenei.

Outside I choose from a row of new yellow taxis, and a clean-shaven young man – let's call him Amir – loads my bags. Tehran is still a long drive north, and we start talking. He has a wife and a 5-year-old girl, but earning a living has been increasingly hard under Iran's sanctions-hit economy.

Amir says he is happy with President Hassan Rouhani and the direction he is taking the country, especially his promises of "constructive engagement" with the outside world and easing restrictions at home. But he didn't vote for Mr. Rouhani, keeping a vow he made after the 2009 election to never cast another ballot.

We pass through a bank of tollbooths, then beneath overpasses with concrete columns lit up in festive colors and festooned with flags and banners. The conversation turns from problems to passions.

"You know, I really am a singer – that is what I love most," says Amir, looking in the rearview mirror to catch my reaction. I am impressed, but not surprised. This is a country where all forms of culture – from painting to cinema – are as deeply engaged in as they are widely appreciated, albeit with many state restrictions.

Amir was in a band that performed traditional Iranian songs, before his marriage cut into his free time.

It is 5:30 a.m., and the city has yet to begin stirring. We drive up the Navvab Expressway through the brightly lit belly of Tehran – and Amir insists on singing to me.

It is a stunning a cappella version of a poem by Saadi, one of ancient Persia's most famous poets. When the last line fades away, Amir explains that it is a conversation with God, a tribute that also describes the order of the world.

"It speaks of how God is everything, all powerful," he says. "And about how small we are by comparison, and how dependent on God...."

• • •

SEEING 'ELVIS'

For a journalist, getting onto the Tehran University campus usually requires letters of approval from authorities, and perhaps an official guide. Or it just requires walking past the guards already overwhelmed by crowds clamoring to hear a speech by Iranian Foreign Minister Mohammad Javad Zarif.

He is the rock star they want to see, the new Elvis of Iranian diplomacy. The goateed, American-educated Mr. Zarif might seem an unlikely hero for students who were barely teenagers when he finished his tour as Iran's ambassador to the United Nations, and then was relegated to the political wilderness during the presidency of archconservative Mahmoud Ahmadinejad.

But today Zarif is the chief diplomat for a president whose election prompted jubilant street celebrations across Iran. And here, in person, is the man who negotiated – during three rounds of marathon talks in Geneva – a nuclear deal that has spurred new hope, by breaking a decade-long deadlock that at times for Iran has risked war and economic ruin. Tougher negotiations await a final accord that will prevent Iran from ever acquiring nuclear weapons, in exchange for lifting crippling sanctions.

Hard-liners are deeply skeptical of the deal, castigating it as a capitulation to the West that endangers Iran's national security. But for most of the students trying to force their way into a packed auditorium, Zarif has brokered a deal just in time to stave off disaster.

On the leafy campus, latecomers cluster tightly at the sliding glass door that leads to the central library, where security guards are trying to prevent more from entering. The door opens slightly, letting in one or two, and those outside surge ahead, forcing the glass facade to bow.

I hold up my camera and press card, like a handful of other journalists who have misjudged the fervent desire of the students to hear Zarif's justification of the Geneva deal, as its salesman in chief.

Guards herd most of the students into a spillover venue with a live video feed. But my interpreter and I join the battle at the door. We finally pop through, finding ourselves at the back of the auditorium. Zarif is just beginning his remarks, making jokes to those standing in the aisles, fanning themselves from the rigors of just getting in.

For their exertion, the diplomat treats them to a discourse on a changing world, and how Iran could take advantage of this "transition time." He says the Geneva deal shows how the government was being successful at "creating security and creating power."

Cheers erupt as Zarif speaks about how military power is giving way to "actual power" of the kind that Iranian voters had shown in electing Rouhani. He also explains why Iran doesn't need nuclear weapons to be strong.

"It is not an honor to be able to destroy the world 100 times," says Zarif, referring to the US nuclear arsenal and the size of its defense budget. "That's why they called it mutually assured destruction: They wanted to create security through lunacy."

A bomb is "not useful" for Iran, says Zarif, but notes that the country will never compromise on its "rights" to peaceful nuclear power.

"We don't even imagine the Islamic Republic with a nuclear weapon," Zarif says, his voice rising. "Even if someone put a nuclear weapon on a platter and gave it to me, I would say, 'I don't want it'.... A nuclear weapon does not create security for us ... it only creates problems and harm and threat for the Islamic Republic."

After two hours, Elvis leaves the building. The case is made. One Iranian security guard apologizes to me for his forceful pushing. As we leave, workmen are already trying to realign the glass door.

• • •

THE MARTYRS

In the elevated climes of north Tehran, the early morning snowfall is heavier. It enforces a magical silence on a city that is otherwise noisy with traffic and managed chaos.

But driving down from the rumpled hills – set against snow-covered peaks – the solitude gives way to frozen sleet and unrelenting gray, which, on the southern outskirts of Tehran, settles grimly on Iran's largest cemetery, Behesht-e Zahra (Zahra's Paradise).

Striking in its expanse and its emotive power, the cemetery is where thousands of martyrs of the Iran-Iraq War in the 1980s are buried. It serves as a pilgrimage site for families carrying flowers for their dead – a place of reflection and love for those most devoted to Iran's revolution and its ideology of resistance.

It has been two decades since authorities dyed the fountain waters here red to emulate the blood of the martyrs – the sacred stuff that Ayatollah Khomeini declared had "irrigated" the Islamic Republic. Today, many of the gravestones still carry, in elegantly carved Persian script, the words "Shahid-e Gomnam" (Unknown Martyr).

But among the rows of memory boxes, full of faded portraits of young men whose survivors have garlanded headstones with fluttering Iranian and Shiite flags, I find Ali. The Iranian student comes weekly to "speak" to three favorite martyrs. He is standing at Grave No. 25, in Section 26, Row 37.

His wet black hair combed back, his hands red with cold, Ali grew up in a traditional religious family. His father was wounded in the war. The 19-year-old has no other connection with these martyrs except, he says, that one day as he was passing he felt the presence of something tug at his arm, and stayed to listen.

"There is one main thing I learned from my father: When there is something more important than your life, you [must be] willing to give your life for it," says Ali, who wears glasses and the embryonic beard of a teenager. "That is why there are so many people here."

Tapping into one of Iran's deepest social divides, Ali says it is "unfortunate" that some secular and Westernized Iranians "do not respect enough" the martyrs here. In the 1980s, when the revolution was still young, some believers falsified their birth dates so they could volunteer for war. Other, more secular and often wealthier Iranians avoided the conflict and disdained the official narrative of fighting Iraq's "infidel" army in the name of Imam Hossein, the 7th-century "Lord of the Martyrs."

That divide remains a fault line in modern Iran – one that shapes every aspect of life, from politics and issues of wealth and class, to how much to compromise with the West, including on nuclear issues.

Ali might seem a natural to join the ranks of the basiji – the ideological militia that helped stamp out the 2009 street protests and before that were the volunteer fighters of the 1980s. But he refuses to sign up. He says too many join these days only to enrich themselves.

"They made changes that me and my family don't believe in," says Ali. He points to his friends, the martyrs: "If the basiji were still like these ones, I could have been a part of them."

Not far away, Khadijeh Ghazikhani, a young woman in a black chador, sprinkles red and pink carnation petals on a slab, kisses it, and explains that she, too, honors three martyrs she has never met. She says they guide her life by the "great things they did, how they lived, and their beliefs."

She notes that they fought not to protect a border, for soil or for money, but for the cause of Imam Hossein. Still, she has seen secular Iranians "not appropriately dressed" come here to pay homage, and even clean some gravestones. "It surprises me sometimes," says Ms. Ghazikhani. "Maybe there are differences about hijab, but [we] have no disagreement over these martyrs."

A boy wanders by, pours water on the stone, and washes it with frigid hands.

"A lot of people get what they want from these martyrs," says Ghazikhani. "Simply said: It keeps the people alive. This is the end of the world here, but I gain energy from it."

• • •

THE DIPLOMATS' SUITE

Iran's downtown Foreign Ministry offices are part of a 90-year-old mix of brick buildings with elaborate facades and Persian tiles, Russian-style balconies, and even replicas of ancient lions wrought in heavy stone. They give the impression of a museum relic rich in glories past.

But this is, in fact, where Iran's diplomatic mandarins toil nonstop to fashion a reemerging regional superpower. Rouhani acquired the "diplomat sheikh" nickname a decade ago, and after the election declared that the surprise result meant a "victory for moderation ... not extremism." He promised to recalibrate Iran's relations with the world. The vote created an "opportunity," he said, and other nations should "take advantage."

Such rhetoric may grab headlines. But the real work of turning that aspiration into reality – from negotiating the nuclear pact and finding mechanisms to ease US-Iranian mistrust, to reassuring anxious neighbors – all takes place here, in simple but well-appointed offices, along hallways with polished marble floors.

Inside one suite, Majid Ravanchi, the deputy foreign minister for European and American affairs, greets a visitor. The career diplomat is one of three Iranians who sat at the table across from the US and five other world powers in Geneva, hammering out the interim nuclear deal.

Mr. Ravanchi wears the Iranian diplomatic uniform: a white shirt buttoned to the top with no collar and no tie (a Western fashion that went out with the revolution), finished with a dark suit jacket, elegantly cut. Among colleagues, his long tenure has earned him the affectionate title "box of secrets."

Ravanchi is articulate in English, smooth, and very aware. Aware of how decades of US-Iranian mistrust can still trump calls for mutual understanding at the hands of hard-liners on both sides who deal in chants of "Death to America" and "Axis of Evil." Aware of how Iran's desire to improve regional ties has rocked two close US allies, Israel and Saudi Arabia, which fear that Iranian hobnobbing with American diplomats might yield a thaw that will disadvantage them.

And he's also very aware that there isn't a lot of time for Rouhani – and those diplomats like him trying to realize their president's vision – to demonstrate that outreach can yield important dividends for Iran.

Since Rouhani's election, "we feel that there is a window of opportunity, an opening to the outside world, and this should be seized," he says. "That is why we have entered the nuclear talks in all sincerity and all vigor to find an amicable solution.... We are not looking at this problem as a way of shifting alignment in the region, or gaining something at the expense of others."

Ravanchi offers me tea with chunks of sugar, Iranian-style, and pistachios in a glass dish. He says Iran's strategy is based on the "conviction that our energies should not be spent on the continuation of tension."

And what of the US? In these offices, nuance mixes with realism and, now, a dash of hope – quite a distance from the routine anti-Americanism that has long characterized Iran's revolution.

"The issues are complex. We have a history that we cannot forget," says Ravanchi. "The important thing is what we have at hand, and that is the nuclear issue. If we can successfully finish this job, I think that will be a big message for both sides that issues of such complexity can be handled with care, and then there are possibilities for future cooperation."

• • •

FRIDAY PRAYERS

Little of that diplomatic nuance is heard in the sermon that washes over the thousands of faithful at Friday prayers, in the heart of the Tehran University campus. America is a big topic here, too – just not usually in a good way.

Since 1979, the words from this pulpit have defined the narrative of the revolution, shaping the ideology of the Islamic Republic's most ardent loyalists.

Security is tight, as usual. Authorities X-ray cameras and bags. They take mobile phones, giving prayergoers numbered slips so they can retrieve them later at buses parked on neighboring streets.

Since I was last here, believers have remodeled the elevated dais in rich blue colors with gold lettering. And in case there is any question that anti-US fervor might have faded with the years, Khomeini's words from decades ago are now emblazoned directly on the front of the lectern. The Persian words state: "We will step over the United States." And then, for the benefit of the foreign media cameras, it says in English: "We defeat the United States."

The anti-Americanism of hard-line Friday prayer leader Ayatollah Ahmad Khatami is legendary, and he often uses bestial imagery, such as a voracious wolf, to convey a point.

Between the religious and political portions of the sermon, an aide changes the setup behind the podium. He places an AK-47 assault rifle on a stand to the left of the ayatollah, so the cleric can grasp the top of the barrel while he speaks. It's a tradition that goes back to the start of the revolution and is meant to symbolize leaning on a weapon against the enemies of God.

In keeping with Ayatollah Khamenei's words that the members of the nuclear negotiating team should be supported, Mr. Khatami affirms that they are indeed "sons of the revolution." But he also issues a warning.

"The diplomatic efforts must continue but must be accompanied by revolutionary spirit and resistance," Khatami says. "This would be like an embodiment of the nation's revolutionary and glamorous 'Death to America' slogan." The crowd chants the slogan loudly, arms raised but with little passion.

Khatami says Iran distrusts Western countries because "they do not understand agreements; their god is [what they gain as] their advantage." Because President Obama, after the Geneva deal, said a military strike remains an option, Khatami asks: "What kind of language is that? We consider Obama as the most impolite president in the entire world."

Few question those points later, when I stop prayergoers on the way out.

"It was the voice of the heart of the people of Iran," says worker Nader Alipour. "This is based on what the US government has done, which has created this view among Iranians. If they view us as an enemy, what can you do with them?"

Little recognition exists here – unlike in the more refined air of the Foreign Ministry – that America's list of grievances is as long as Iran's. "Obama is speaking beautifully, but behind the scenes we see they are doing something else.... We want honesty," says Ahmad Reza, a bearded English teacher in a woolen hat. Americans are "lovely" and "honest people," but "I am at the front line of those who chant 'Death to America.' Americans should look to the reasons why."

"Great Satan," interjects another man, before pushing off from the cluster that has gathered around me.

A third man says Americans and Iranians "both believe in God, in Judgment Day, and in the holy prophets," but that the US government "doesn't believe in those things."

One person challenges him. "What? They have 'In God We Trust' written on the dollar."

Another man, as he leaves, tells me: "Your eyes are precise."

What does that mean – is he my friend? Am I his enemy? After Friday prayers, neither really. For now, just acquaintances.

• • •

YES WE CAN, TOO

Hossein Dehbashi is part James Carville, part Martin Scorsese. He uses video to move people about politics – and his latest creation is sweeping through parts of Iranian society.

Mr. Dehbashi is the director of "New Voyager," an inspirational political video created to mark Rouhani's first 100 days in office. The short black-and-white film mirrors the techniques and even the sequences of the popular "Yes We Can" video about Mr. Obama.

But it pushes the boundaries of what is acceptable in Iran and has become an early cultural flash point for Rouhani. It shows a high degree of openness from the new president, who put it on his website, where it garnered half a million hits in the first 48 hours.

Yet it has also provoked anger from hard-liners who say Rouhani and his videographer have gone too far. Already a group of motorcyclists has menaced Dehbashi's parents' house, spray-painting accusations on the walls.

I met Dehbashi one night intending to interview him over dinner. He picked me up in his four-wheel-drive vehicle and started talking so passionately about the video that I immediately began taking notes. The restaurant turned out to be closed, so we pulled over along busy Keshavarz Boulevard and continued talking.

"I'm not just happy because this video was popular, but because now we have a president who understands the power of art," says Dehbashi, who earlier had directed two official campaign videos for Rouhani but made this one on his own.

Political videos are an art form in Iran. They have been honed from the earliest years of the revolution when Khomeini declared that "of all the organs of propaganda," radio and TV were "more important than schools" in shaping popular perceptions.

Dehbashi's earlier campaign videos highlighted the "prudence and hope" slogan of the centrist cleric, who would unexpectedly trounce a slate of conservative rivals. Yet "New Voyager" is the most emotive, with a nod toward Iranian history and a message of national unity.

Iranians of all ages perform to Rouhani's own words, in a speech given the day Khamenei formally endorsed him as president (Aug. 3). "Let the space and opportunity to serve the people open up to all Iranians whose hearts are tied to this land," Rouhani intones. "Let our hearts be cleansed of resentment...."

The film breaks taboos by showing images of men and women singing (the women sometimes with a glimpse of hair visible), and of on-screen playing of musical instruments – all things officially forbidden. Singers also echo Rouhani's words in the languages of Iran's Baloch, Arab, Azeri, and Kurdish minorities, as he speaks of the "compassionate face of Islam" and "rational face of Iran." He asks for the Lord's help to avoid "autocracy" and not "shut the mouths of rivals and critics."

The film was powerful enough that hard-liners criticized it three times in a week in the Kayhan newspaper. The attacks on Dehbashi's parents' house were even more alarming. They recalled incidents in the late 1990s and early 2000s when vigilantes assailed reformists at rallies and cultural events, often with chains and clubs. Vigilantes were also deployed to crush protests after the disputed election of 2009.

When Rouhani himself watched the video for the first time, unaware of who had directed it, but very aware of its rule-breaking, he insisted on seeing it four times. Then he said: "It is excellent. This is exactly the thing that I want. Who made it?"

By posting the video on his website, Rouhani showed, in Dehbashi's view, that he is a modern man. "What does it mean? That [Rouhani] says: 'I don't agree with all the rules [of state-run media]," says the director. "The main target: We wanted to show [Rouhani] is not a conservative person."

Dehbashi vows to resist the pressure from hard-liners. "It's obvious they are angry with [Rouhani], not me, but because of the honeymoon they can't be so open about it," he adds. "I am nobody, but if they can make me afraid, others like me will be also."

• • •

IRAN'S GENERAL MOTORS

It's after dawn, a workday, and the rumble of cars is picking up along a wide central avenue in Tehran, revealing a new era of determined organization that I have not seen in this vast metropolis before.

At bus stops, screens display real-time schedules and detailed maps. Some street corners have tubs of salt for snowy days. Drivers now use electronic meters to process parking slips. Beautification schemes are everywhere.

On a bench, in a narrow roadside park and partly hidden behind a hedge, a young couple shares a moment of intimacy, holding hands and sitting close before their day begins.

Nearby, a city worker – part of an army in bright green and yellow overalls that attempts to keep Tehran tidy – sweeps up leaves. Another paints exercise equipment, taking advantage of the sunlight after days of storms.

All the activity partly masks how much Iran's economy has shrunk and its currency has tumbled under the weight of US-orchestrated global sanctions. They have targeted Iran's lifeblood – its oil exports – as well as its central bank and virtually all financial transactions.

So Iranians complain, and want improvement. "Your country has impoverished me!" an Iranian friend told me, only half-joking. That is one reason so many here celebrated the Geneva nuclear deal and its promise of "modest" sanctions relief.

Where is the easing likely to be felt first – the Exhibit A benefits for Rouhani to show that Iran's engagement with the West is worthwhile? One place is an hour's drive west, past ribbons of traffic-choked highways, at the mammoth Iran Khodro Company manufacturing plant, which can churn out as many as 42 cars per hour.

Iran's car industry has been hit especially hard by sanctions, with production falling 72 percent this year from a 2011 output of 1.6 million vehicles, reports The Associated Press. As many as 100,000 layoffs are tied to sanctions.

Still, this factory is one of the largest of its kind in the Middle East and North Africa, with tens of thousands of employees. The facility is a city unto itself. It has named streets inside and even its own police patrols.

Entering the cavernous halls, one is struck by the smell of machinery and oil, the sight of sparks flying as metal is tack welded, of robots moving with relentless precision as silver chassis slide by. And there is a feel of constant activity. Workers in blue Khodro uniforms clearly take pride in their production lines.

While I'm at the plant, company officials give a tour to a group of schoolgirls. They snap pictures and giggle, their black chadors fluttering as they pass.

The Geneva deal states that US sanctions on Iran's auto industry and its "associated services" are to be suspended, which is good news for a company with a long history of foreign partnerships from Mercedes-Benz to Peugeot, Renault to Suzuki.

"Every year the technology and know-how is changing," says Hossein Najjari, the managing director for Khodro's automotive parts supply wing, SAPCO. "In order to keep market share, we need to update ourselves ... so the most important benefit of this [Geneva] agreement for the auto industry is opening communications with all the world."

That means a "new generation" of technologies that includes hybrid, electrical, and safety advances. As sanctions have taken hold, the company has been able to supply its basic needs, but not the most advanced parts.

European companies are already signaling their post-sanctions interest, if the Geneva deal holds up. And it might not be long before American ones, too, step back in after decades away.

On the factory floor, that day won't come soon enough for some. On the assembly line where he mounts car dashboards, one worker smiles broadly after hearing where I'm from – and alerts all the men down the line.

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"America," he says, holding up white-gloved hands, as if his team had scored a goal. "I love it!"

• Scott Peterson is the author of "Let the Swords Encircle Me: Iran – A Journey Behind the Headlines." Follow him on Twitter at @peterson_scott.