TEHRAN (Reuters) - The man posing the biggest political challenge to Iranian authorities in three decades is a veteran of the Islamic revolution and an unlikely champion of reform.
Mirhossein Mousavi's supporters have taken to the streets in their tens of thousands to protest official results of a June 12 presidential election, in Iran's most serious unrest since the overthrow of the U.S.-backed shah in 1979.
Mousavi advocates better ties with the West, enjoys the support of leading Iranian reformists, and broke new ground in the socially conservative country by campaigning openly alongside his wife, a prominent academic.
But the grey-bearded, bespectacled candidate defeated by hardline president Mahmoud Ahmadinejad is also an establishment figure, who served eight years as prime minister in the first turbulent decade of Iran's clerical rule.
While he supports detente with Iran's Western foes, he rejects their main demand -- that Tehran halt uranium enrichment which the United States fears could be used to make bombs. Iran says its nuclear programme is for peaceful power generation.
His calls for responsible economic and foreign policy, and an easing of social restrictions on women, suggest modest ambitions for incremental change rather than shaking the foundations of the Islamic Republic.
During campaigning he sought to win over conservative voters by urging a return to the "fundamental values" of the republic's founder, Ayatollah Ruhollah Khomeini.
U.S. President Barack Obama, who has sought to engage Iran after nearly 30 years of diplomatic rift, said this week the difference between Mousavi and Ahmadinejad "may not be as great as has been advertised".
CHANGE OF IMAGE
But Mousavi, 67, who spent 20 years out of the political spotlight before launching his presidential campaign, captured the imagination of young and more affluent Iranians -- many of them not old enough to remember his 1981-1989 premiership.
In the days leading up to the vote tens of thousands poured onto Tehran's streets for vibrant and noisy pro-Mousavi rallies, a prelude to the post-election protests which followed official results showing a resounding victory for Ahamdinejad.
While a change of president would not mean a major change in policy -- which is decided by Supreme Leader Ayatollah Ali Khamenei -- Mousavi offered a significant shift of tone.
He promised to soften the "extremist" image that Iran earned abroad under Ahmadinejad and to continue talks with major powers on the nuclear issue, in contrast to Ahmadinejad who has ruled out talks.
In a heated television debate with Ahmadinejad a week before the vote, he said Iranians had been "humiliated around the globe" under the hardline president.
He also took aim at Ahmadinejad's questioning of the Holocaust, saying it only served to increase sympathy for Israel -- a country Iran does not recognise.
Mousavi is backed two former presidents -- reformist Mohammad Khatami and powerful conservative Akbar Hashemi Rafsanjani -- demonstrating a wide coalition of support.
U.S. Senator John Kerry said his early years of service to the revolution meant he could advocate a more conciliatory approach to the United States without appearing to be an agent of the West.
"His political legitimacy comes from his revolutionary credentials for helping overthrow an American-backed shah, a history that today helps protect protesters against accusations of being an American 'fifth column'," Kerry wrote in the New York Times on Thursday.
Mousavi, the son of a tea merchant, was born in Khameneh, the same town as Khamenei, in 1941. He is also a distant cousin of the supreme leader who will play the decisive role in resolving the election dispute.
A painter and architect, specialising in Islamic design, he is married to Zahra Rahnavard, a former university chancellor and presidential adviser under Khatami.
Rahnavard campaigned actively by his side ahead of the election, promising her husband would fight for women's rights and political freedoms, prompting suggestions that she could take on a pioneering 'first lady' role in Iran.
"I am not Michelle Obama," she told a pre-election news conference. "But I do respect all women activists wherever they are in the world."