When thousands of Iranians flocked to the streets to celebrate the signing of the historic accord, it was Zarif's name they chanted.

As the Islamic republic's top diplomat, Zarif had led the negotiations between September 2013 and July 2015 which led to the deal, heralded as ending Iran's international isolation.

But initial elation came crashing down in May when US President Donald Trump pulled out of the agreement and reimposed crippling sanctions, hitting Iranians hard.

Zarif has for years come under fire from Iranian conservatives, who resented his close relationship with his US counterpart, John Kerry, during the nuclear talks.

"We were more worried by the daggers that were struck from behind than the negotiations," he told the Jomhoori Eslami newspaper in a February 2 interview.

"Internal pressure wore me down both during and after the talks."

Spending more than a third of his life in the United States and speaking flawless English have made him a hate figure for hardliners in his own country.

That is despite his status as a veteran of the Islamic revolution that toppled Iran's US-backed monarchy in 1979.

Those who last year failed to impeach Zarif continued their criticism of him Tuesday following the diplomat's decision to resign, although some lawmakers stood by him.

- 'Mr Ambassador' -

Raised in a religious family in Tehran, the two-time ambassador to the United Nations revealed in a best-selling memoir, "Mr Ambassador", that he did not listen to music until he was 15.

His involvement with politics goes back to his teenage years, when he attended secret meetings in the run-up to the revolution.

When he was 16, his parents sent Zarif to California after the Shah's regime threatened to arrest him.

There, he joined the Islamic Student Association and made many friends who later became political figures in Iran.

Among them was the brother of Akbar Hashemi Rafsanjani, president from 1989 until 1997.

He held on to his devout ways. When he lived in the United States in the 80s, his wife, then a staunch revolutionary, did not allow him to buy a television for nearly 10 years, he said.

She later became a follower of Mohammad Esmail Dulabi, a mystic whose teachings changed her into "a quiet person filled with patience and tolerance".

"The new version had the greatest influence on our family," Zarif said. They have two adult children.

Following the seizure of the US embassy in Tehran by Islamist students in 1979 and the subsequent cutting of ties with Washington, Zarif was sent to shut down Iran's consulate in San Francisco.

He then studied international relations and earned a PhD from the University of Denver, writing a dissertation on "sanctions in international law".

Zarif has decades of experience as a negotiator.

He was a member of Iranian delegations which negotiated a ceasefire ending eight years of war with Iraq in 1988 and secured the release of American hostages in Lebanon in 1991.

In the late 1980s, he joined the Iranian delegation at the United Nations and was ambassador from 2002 to 2007, during the presidency of reformist Mohammad Khatami.

He was sacked by hardline president Mahmoud Ahmadinejad.

But when the more moderate Hassan Rouhani became president in 2013, Zarif was rehabilitated and picked to head the foreign ministry.

However, despite his fervent efforts to save the nuclear deal, Zarif hinted that internal strife had made his position untenable.

In an interview published Tuesday, he told Jomhoori Eslami: "Everything will be lost when there is no trust in the manager of foreign policy."

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