Ebrahim Yazdi, who has died aged 86, was a longstanding member, and from 1995 leader, of the Iranian opposition party the Freedom Movement of Iran (FMI). The FMI was founded in 1961 by Mehdi Bazargan, the champion of Islamic modernism in Iran, who became Ayatollah Ruhollah Khomeini’s first prime minister after the Islamic revolution of 1979 that overthrew Shah Mohammad Reza Pahlavi. Yazdi influenced the early policies of the Islamic revolution but, as a moderating force within it, lost ground to the hardliners who increasingly won Khomeini’s support. The FMI was soon suppressed by the new regime, but the party continued to be tolerated and has not been banned even to this day.

He first rose to prominence in 1978 as a close associate of the exiled Khomeini and presented the Ayatollah to the western media not as a fundamentalist, but as a constitutionalist who, once back in Tehran, would step back from power to make way for liberal democracy. Some suspected that Khomeini was exploiting the democratic credentials of the FMI and its leaders to delude the west into believing that he genuinely sought a western-style democracy.

Yazdi became foreign minister in Iran’s interim post-revolutionary government but his tenure was short-lived. On 4 November 1979, 52 American hostages were seized in the US embassy in Tehran and shortly afterwards Khomeini appeared on television endorsing the action. In response the entire cabinet, including Yazdi and Bazargan, resigned in protest. The result for Yazdi was that he spent the rest of his political career in opposition; he continued to speak out against constitutional and civil rights abuses and between 2009 and 2011 was arrested several times after criticising Mahmoud Ahmadinejad’s dubious re-election as president.

The son of a merchant trader, Yazdi was born in Qazvin in the north-west of Iran. He studied pharmacology at the University of Tehran, and worked in Iran until 1959, when he moved to the US to attend the Massachusetts Institute of Technology. He joined the militant opposition to the shah and in 1961 became a member of the newly formed FMI. He later joined the staff of Fairleigh Dickinson University in New Jersey and was granted US citizenship. During this period he made contact with US experts on Iran in the hope of influencing US policy towards the shah.

In 1963 Yazdi formed the Association of Iranian Islamic Students to propagate opposition to the shah among Iranian students abroad. Later he visited Cairo and met two men who would become key figures in the Islamic revolution, Mostafa Chamran, the defence minister after the revolution, and Sadegh Ghotbzadeh, later the foreign minister.

In 1967 Yazdi took up a research post at Baylor medical college in Houston, Texas. In 1972 Khomeini named him his personal representative and intermediary with US officials. Six years later, when Khomeini decided to leave Iraq, Yazdi was there as the Ayatollah tried to enter Kuwait and was refused entry. The cleric eventually arrived in France, where Yazdi had set up a headquarters for him at Neauphle-le-Château, outside Paris.

In February 1979 Yazdi returned to Iran as the head of Khomeini’s entourage and was named the deputy prime minister for revolutionary affairs in Bazargan’s interim government. But liberals were gradually sidelined by the clergy, who moved in to occupy key positions in the newly created revolutionary institutions. When the Revolutionary Guard was formed in 1979 to impose some discipline on armed groups and as a clerical counterweight to the army, Yazdi vied for influence with them. However, by 1981, during the clampdown on moderates and the arrest – and later, execution – of the moderate Ghotbzadeh, Yazdi was criticising the now ruling Islamic Republic Party for its “Stalinist and unIslamic methods”.

After the Iraqi invasion of Iran in 1980, Yazdi fully supported the Iranian war effort, but he opposed its continuation after the Iranian victory in Khorramshahr in 1982. Over the next six years, Yazdi and others in the FMI issued open letters to Khomeini urging an end to the war. This led to the firebombing of Yazdi’s residence in Tehran.

In 1989 Khomeini died, to be succeeded as supreme leader by Ayatollah Ali Khamenei. In 1995 Bazargan died and the leadership of the FMI passed to Yazdi. The following year the regime cracked down heavily on the movement. Yazdi was arrested and imprisoned in Tehran. Even after his release, he was barred from leaving the country for many years and summoned on a regular basis to answer questions before the revolutionary council. In 2008 he was accused of “attempting to convert the rule of velayat-e faqih (guardianship of the jurist) into democratic rule” and in 2011 he was convicted on charges of “attempting to act against national security”. Suffering from cancer and a heart condition, he was sentenced to eight years in jail.

Eventually released on health grounds, he died in Izmir, Turkey, where he was receiving treatment for pancreatic cancer after being denied a US visa.

He is survived by his wife, Sourour, four daughters and two sons.

• Ebrahim Yazdi, politician, born 3 April 1931; died 27 August 2017