* Mousavi still defiant after disputed June election
* Loyalist of Iran's revolution becomes opposition symbol
BEIRUT, Aug 4 (Reuters) - Mirhossein Mousavi's credentials as a loyal servant of Iran's revolution may help explain why he has escaped arrest for leading protests against an election he says was stolen to keep President Mahmoud Ahmadinejad in power.
The 68-year-old moderate politician is an unlikely hero for Iranian voters who switched from euphoric anticipation that he might win the June 12 poll to disbelief and anger when his fiery opponent was abruptly declared victorious by a wide margin.
Ahmadinejad will be sworn in by parliament on Wednesday, when the Revolutionary Guards and Basij militiamen will likely seek to foil any repeat of the post-election street unrest in which at least 20 people were killed and hundreds were detained.
Mousavi may lack charisma, but not courage. He has castigated the authorities for their handling of the election and its tumultuous aftermath. He has even defied his relative, Supreme Leader Ayatollah Ali Khamenei, who overtly backed Ahmadinejad.
"Mousavi became a national hero by default," said Mehrzad Boroujerdi, an Iran scholar at New York's Syracuse University.
"What has endeared him to the public is the fact that contrary to former President (Mohammad) Khatami, who would be reluctant to stand up to Khamanei and others, Mousavi has stuck to his guns," Boroujerdi said. "In the process he has forced many cautious leaders to come out in support of the movement."
Mousavi has previously demanded the elections be anulled, but may need a new goal once Ahmadinejad is reinstalled.
"The plan should be to call into question the legitimacy of Ahmadinejad's administration at every turn, through civil disobedience, and also to press for some revisions to the constitution," Boroujerdi suggested.
Farideh Farhi, an Iran specialist at the University of Hawaii, said defiance had been Mousavi's main card so far.
"Given the extent of popular mobilisation, he simply cannot back down at this point and he has not done so," she said.
"His tactic of shifting the focus to the violence that has ensued since the election and systematic violation of the law by the security forces is his only way of ... supporting the protesters and making sure they know he is with them."
ALLIES ON TRIAL
Mousavi has yet to unveil a promised new political front with his reformist and pragmatist allies, perhaps partly because so many leading figures are in jail, including 100 whose trial for inciting unrest began on Saturday and resumes on Thursday.
Prime minister during Iran's 1980-88 war with Iraq, Mousavi was active in the 1979 Islamic revolution that toppled the Shah and continues to proclaim his fidelity to revolutionary ideals.
During his election campaign he wooed conservative voters by urging a return to the "fundamental values" of the Islamic Republic's founder, Ayatollah Ruhollah Khomeini.
Since the disputed vote, Mousavi has enjoyed vocal support from Khatami and reformist candidate Mehdi Karoubi, as well as the tacit backing of former President Akbar Hashemi Rafsanjani.
His opponents undeniably control the means of coercion, but the tumult after the election has alienated some conservatives and even opened up rifts among Ahmadinejad's hardline allies.
Mousavi has not challenged Iran's system of clerical rule, even if many who support him may be chafing for broader change.
Grey-haired, bearded and bespectacled, Mousavi has the air of a calm intellectual. At the height of the street protests, he greeted the hundreds of thousands who turned out for him, but never sought to fire them up for fiercer confrontations.
Now he is one of several leaders in a broad-based movement that has largely avoided energy-sapping internal conflicts that might divert its momentum in the struggle with the authorities.
Iranian hardliners accuse Mousavi and his allies of working with foreign enemies, especially the United States and Britain, to instigate a "velvet revolution" against Islamic rule.
At least for now, the opposition seems set not on toppling the system, but on trying to make it responsive and accountable, an effort framed by Mousavi and others in revolutionary ideals.
"The situation has gone beyond the demands of one man or the leadership of one man," Farhi said. "Systemic change is something that is down the road and very much reliant on the continuation and expansion of popular mobilisation."
Mousavi offered no detailed political or economic programme during his election campaign, in which his wife Zahra Rahnavard played a striking role. But he promised a more open cultural atmosphere and an effort to reduce foreign policy tensions.
"He is giving the impression that he is a principled man genuinely outraged by what has happened to Iran," Farhi said, adding that he had no real choice but to stand firm or risk losing legitimacy in the eyes of those protesting for him.
Viewed as a traitor by hardliners, Mousavi had shunned politics for two decades before his run for the presidency thrust him into the unaccustomed role of opposition symbol.
"He is driven by his rock-solid revolutionary convictions, his antipathy for Ayatollah Khamenei, and his moral debt to those who voted for him and those who lost their lives in the post-election milieu," Syracuse's Boroujerdi said.
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