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TEHRAN, Iran — The most significant protests in eight years are rocking Iran, with state media reporting Tuesday that the death toll from clashes between demonstrators and security forces had reached at least 20.

Offering his first comments during the six days of unrest, Supreme Leader Ayatollah Ali Khamenei on Tuesday accused the "enemies of Iran" of meddling in the country's affairs.

In recent events, enemies of #Iran have allied & used the various means they possess, including money, weapons, politics &intelligence services, to trouble the Islamic Republic. The enemy is always looking for an opportunity & any crevice to infiltrate &strike the Iranian nation. pic.twitter.com/HIXtudRAue — Khamenei.ir (@khamenei_ir) January 2, 2018

Hundreds of people have been arrested and activists are taking the rare step of publicly criticizing the country's religious leaders. They are the largest protests since the country's disputed 2009 presidential election — which triggered the so-called Green Movement.

State TV reported Tuesday that six people had been killed during an attack on a police station in the town of Qahdarijan. It reported that clashes were sparked by rioters who tried to steal guns from the police station.

Iran's reformist President Hassan Rouhani appeared to acknowledge the seriousness of the protests Monday when he tweeted that "the authorities must pay attention to the people's demands."

What's happening?

The protests started out as local rallies against Iran's economic problems but have since spread in both geography and scope.

Iranians last week took to the streets of Mashhad, the country's second-largest city, to protest high inflation and the rising prices of everyday goods.

This frustration is hardly new. Many people expected that their financial situation would improve after Iran signed a nuclear deal in 2015 with the U.S. and five other world powers.

The country agreed to limit its uranium enrichment in exchange for sanctions being lifted. The economy has improved — but there is widespread anger that the benefits have not trickled down to ordinary citizens.

The demonstrations spread to Tehran on Saturday, with people chanting anti-government slogans and tearing down political posters.

The next day, officials said that two protesters had been shot dead overnight in the western city of Doroud. Authorities denied they were killed by police, instead blaming foreign agents and terror groups for the deaths.

State TV reported Monday that another 10 people had been killed during clashes. Clashes overnight between protesters and security forces killed nine others, state television reported early Tuesday.

What started as an isolated economic grievance appears to have morphed into a wider expression of dissatisfaction with the government.

"I think it's far more serious than we anticipated a few days ago," said Sanam Vakil, an associate fellow at London's Chatham House think tank.

How has the government reacted?

Since the Islamic Revolution in 1979, Iran has been a religious conservative society where many aspects of public life, such as freedom of expression, the press and public assembly, are restricted.

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On Sunday, Rouhani, who is seen as a reformist, said that while people had the right to demonstrate he condemned "violence and destruction of public property."

Iranian officials have also partially blocked Instagram and the messaging app Telegram, which was used to distribute information and images of the protests.

Videos on social media purported to show some protesters shot dead by government security forces, although NBC News could not verify the videos or details of the shootings.

Hundreds of people have been arrested and security has been overseen by the regular police. As yet neither Iran's feared volunteer militia force, known as the Basij, nor the revolutionary guard have not been deployed to crush the dissent.

"Whether it's going to happen slowly or more immediately, I think the government is going to start cracking down," said Vakil at Chatham House. "We've already started seeing it — people have been killed, there have been arrests and curtailing access to the internet. And if people don't stop protesting that will only increase."

However, she also predicted that the government would have to offer some sort of carrot, perhaps in the form of economic concessions, alongside this stick.

Over the weekend, Iran's hardliners, some of whom are critical of the more moderate Rouhani, took to the streets to defend Khamenei and others in planned demonstrations of support for the regime.

Why are the protests significant?

The unrest is the biggest public challenge to Iran's status quo since 2009, when a disputed presidential election saw millions take to the streets in what became the Green Movement.

The current protests at far smaller, and there are other significant differences.

In 2009, it was more of a middle-class uprising, involving demonstrators who had livelihoods and would go to work the next morning after protesting the night before.

People who lived in more affluent parts of Tehran would shout "Allahu akbar," meaning "God is great" in Arabic, or "death to the dictator" from their rooftops and balconies. Their focus was on reform rather than revolution.

Today, however, the middle classes do not appear to be participating in a similar way. The activists who are taking to the streets appear to be more working class, radical and provocative than in the past, and they don't appear to be chanting for any leader.

There is an expression in Iran that says "the knife has hit the bone." This is being used to describe these demonstrators, who feel they have little left to lose.

Also unlike those behind Green Movement, today's activists are not chanting the names of any opposition leaders or wearing green bracelets in support of reforms.

Without the unifying presence of a central figurehead or cause, "it's really hard to see where this is headed," Vakil said.

Rouhani, the current president, is also seen as more moderate than his predecessor in 2009, Mahmoud Ahmadinejad, something perhaps reflected in his statement this weekend permitting some criticism of the government.

How has the U.S. reacted?

President Donald Trump, whose travel bans blocked Iranians from getting U.S. visas, reiterated his support for the protests Tuesday, tweeting that: "The people of Iran are finally acting against the brutal and corrupt Iranian regime."

"All of the money that President Obama so foolishly gave them went into terrorism and into their 'pockets,'" he added, referring to the 2015 nuclear deal. "The people have little food, big inflation and no human rights. The U.S. is watching!"

A day earlier, Trump had tweeted that Iran was "failing at every level" and that it was "TIME FOR CHANGE!" Over the weekend, Trump tweeted similar thoughts.

Big protests in Iran. The people are finally getting wise as to how their money and wealth is being stolen and squandered on terrorism. Looks like they will not take it any longer. The USA is watching very closely for human rights violations! — Donald J. Trump (@realDonaldTrump) December 31, 2017

Rouhani appeared to respond to Trump’s criticisms in his comments Sunday.

"Government and people solve problems together. One who calls the Iranian nation a terrorist does not have the right of compassion for our people," the Iranian president said.

However, it is unclear if Trump's support will give the protesters a boost of encouragement, or whether it will be used by Iranian hardliners as evidence that foreign powers are in fact behind the disruption.

In 2009, marchers chanted "Obama, Obama either you are with us or you are with them," a call largely ignored by the former president, who didn’t want to jeopardize the nuclear deal.

The crowds this time in Iran have not called for support from Trump — so far. Vice President Mike Pence tweeted that "America will not repeat the shameful mistake of our past when others stood by and ignored the heroic resistance of the Iranian people."

In the main, Iranians are already skeptical over the president's refusal to re-certify the nuclear deal, while Iran's government has often used comments by U.S. officials to dismiss protests as a sign of foreign interference in its internal politics.

"The people of Iran give no value and credit to Trump," Foreign Ministry spokesman Bahram Ghasemi said Saturday. "The powerful people of Iran don't waste their time with opportunist and meddlesome slogans of American officials."

Ali Arouzi reported from Tehran. Alexander Smith reported from London.