Iran’s nuclear negotiators received a hero's welcome at home after sealing a breakthrough deal in Geneva, their convoy mobbed by hundreds of jubilant Iranians waving flowers and flags – the first images of popular support that will be used to sell the deal at home.

Despite some sniping from hardliners that Iran gave away too much for too little in Geneva, for most Iranians the deal brings a sense of relief and joy at the prospect of an improved economy and changing relationship with the outside world.

The six-month deal, reached in the early hours Sunday, halts the advance of Iran’s nuclear program in exchange for a partial lifting of sanctions as the first step to a comprehensive final agreement.

In Tehran, Foreign Minister Mohammad Javad Zarif was praised as an “ambassador of peace” for tireless efforts during three rounds of Geneva negotiations with world powers since October. Chants rang out, “No to war, sanctions, surrender and insult,” the AP reported.

Both Iran and the P5+1 group (the US, Russia, China, Britain, France, and Germany) have declared victory, and portrayed the other side as receiving limited and “reversible” gains – a bid to preempt domestic critics.

In Iran, the government campaign to sell the deal to the public immediately began at the highest level, with a very public exchange of letters between Supreme Leader Ayatollah Ali Khamenei and the centrist President Hassan Rouhani, who has been in office just three-and-a-half months. A blessing from Mr. Khamenei can go a long way to silence potential critics in Iran; a less supportive message would have been seen as a license to attack and undermine President Rouhani.

In his letter, Mr. Rouhani praised the “divine success” under Khamenei’s guidance. He said Iran’s negotiating team had “showed the big powers can be urged to respect the Iranian nation’s rights” and “through logical and reasonable presentation [can achieve] respecting all [Iran’s] principles and red lines,” according to a translation by the Fars News Agency.

In return, Khamenei expressed an unprecedented level of support for his negotiators, the highest for any during a decade of high-stakes nuclear diplomacy with the West.

“Achieving what you have written is worth appreciation and praise to the nuclear negotiating team…and can be the basis for future smart moves,” Fars quoted Mr. Khamenei as writing. “God willing, resistance against greediness (of the other side) should always remain as an indicator [of] a correct path.”

Amid chaotic scenes at the airport Sunday night, Mr. Zarif posed for a few photos, in one surrounded by a crowd of young female supporters, indicative of the widespread support of young Iranians who helped give Rouhani a first-round victory in June's elections.

Most newspapers covered their front pages Monday with the news, including the reformist Shargh, which showed a large photograph of Zarif clasping hands with US Secretary of State John Kerry at the signing ceremony, both men smiling, and a US flag in the background.

It was a rare sight for senior officials of these two nations, and rarer still to be on Iranian front pages. In the past, Iran has called the US the "Great Satan," and American flags are still routinely burned after Friday prayers. Washington used to refer to Iran as part of an "Axis of Evil," and still considers the Islamic Republic the "most active" state sponsor of terrorism.

“All the measures that we will take, the confidence-building measures, are reversible, and they can be reversed fast,” Zarif told state TV in an interview at the airport, echoing senior US officials about the reversibility of sanctions relief. “Of course, we hope we don’t have to do this.”

Zarif affirmed that the Geneva deal “stipulates twice that there will be no resolution of the nuclear dispute without a nuclear enrichment program inside Iran” – which has been a perennial red line for Iran, and a key sticking point in all rounds of talks.

Writing on his Facebook page in Persian, Zarif later thanked Khamenei for his praise in the letter, “where he bestowed grace upon his children as before.” Public messages of support “brought tears of joy into our eyes as your modest serving staff,” said Zarif, according to the Mehr News Agency.

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Zarif also warned critics to tread carefully, and not give Israel, which has slammed the deal as akin to appeasement, an opening for derailing it.

“We would welcome constructive and benevolent criticism, but we expect in return that our friends [keep] in mind two considerations; fairness of judgment and more important, national interests,” Zarif wrote. “You should be alert that Zionists and other warmongers [are] all extremely on edge and they would spare no pretext and device to bring a deal dubbed a deal of the century for Iran, into nothing."