He's gone from colorless insider to political rock star — a graying, bearded veteran of the Islamic regime who now stands at the forefront of a youth-driven movement fighting for change.

Despite his newfound fame, Mir Hossein Mousavi still works out of his old office at the Iranian Art Academy and lives in the same unassuming brick home in a middle class district of Tehran as before, according to an aide.

Only now, he travels with armed guards provided by the very government he is challenging.

When he appears in public, such as at an opposition rally Thursday in Tehran, crowds surge around his car, chanting his name, according to witnesses in the Iranian capital.

It's unclear what has propelled this calm, deliberate architect and artist — who twice refused to seek the presidency — into a confrontation with the ruling establishment of which he was once a part.

Click to view photos from Iran.

Click here for FOXNews.com's Iran content center.

Nor is it clear how Mousavi will respond if the opposition movement transforms from a campaign against alleged fraud in the June 12 election into a major challenge against the core values of the Islamic Republic — that senior clerics have the final say on major issues.

Even during the election campaign, Mousavi was less critical of President Mahmoud Ahmadinejad than another challenger, former parliament speaker Mahdi Karroubi, who received only a fraction of the vote.

Associates say the real firebrand in the Mousavi family is his wife, Zahra Rahnavard, a prominent professor who campaigned by his side.

For years, he remained out of the political limelight, painting pictures — mostly with religious themes — and designing buildings, including two universities, a mosque, a museum and a shopping center.

Nevertheless, a 67-year-old figure nearly devoid of personal charisma has become the champion of a generation inspired by the hope of change, organizing protests with technologies such as mobile phones and Internet that didn't exist when their parents overthrew the U.S.-backed shah in 1979.

In his new role, Mousavi has displayed a common touch — something Ahmadinejad has also sought to portray in an effort to identify with millions of impoverished Iranians.

On Thursday, crowds cheered as Mousavi, dressed in a black coat and trousers, climbed on top of his SUV, addressing his followers through a loudspeaker rather than mounting a stage.

Although a number of his followers have been arrested, aides insist Mousavi himself has maintained his old routine, even as his challenge to the powerful clerical establishment is growing.

"Mousavi goes to his regular job as the head of Iran's Art Academy and lives with his family in the same place he lived before the election," said Qorban Behzadian Nejad, head of his campaign headquarters.

"At the same time he pursues his activities for nullification of the election."

Much of Mousavi's appeal among Iranians eager for change probably stems simply from the fact that he is not Ahmadinejad, a hard-liner who has failed to deliver on economic promises and who seems to relish provocative statements — from calling protesters "dust" to denying the Holocaust — that stir controversy at home and abroad.

Hard-liner was a once a term also used to describe Mousavi during the early years of the Islamic Republic, which was established after a cleric-led uprising toppled the monarchy in February 1979.

Like many educated young Iranians, Mousavi was drawn into the Islamic movement by the writings of Dr. Ali Shariati, an Iranian sociologist who tried to mesh Islamic and socialist ideals until he died in Britain under mysterious circumstances in 1977.

At the core of Shariati's writings was the notion that religion should be used as an instrument for social justice and change.

Those ideas inspired many idealistic young Iranians to join Ayatollah Ruhollah Khomeini in the campaign to oust Shah Mohammed Reza Pahlavi in 1979.

Mousavi played an active role in the 1979 revolution, then served as foreign minister and later prime minister of the new Islamic republic from 1981 until 1989.

For most of those years, the nation was at war with Iraq, and the government displayed little tolerance for dissent. Critics faced arrest and execution as traitors.

During that period, Mousavi's office also approved the beginnings of a secret nuclear program, which has since put Iran on a collision course with the United States and its Western allies.

But he also won praise for his management of Iran's economy and for trying to end the country's international isolation — policies that often put him at odds with President Ali Khamenei, now the supreme leader and ultimate authority in the country.

That has led some analysts to question whether Mousavi is motivated as much by the prospect of settling old scores with Khamenei as with promoting fundamental change in Iran.

After the post of prime minister was abolished, Mousavi left the public stage, although he continued to serve on key governmental councils.

He spent most of the next decade working as an architect and helping rear his three daughters.

In 1997, reformists urged Mousavi to run for president but he refused.

Instead the reformers turned to an obscure cleric, Mohammed Khatami, who was elected by a landslide but found himself with little power to confront the conservative religious establishment.

Mousavi also declined to run in the 2005 election, which Ahmadinejad won.

This year, in a reversal of fortunes, Mousavi accepted the challenge after Khatami took himself out of the running. With that came a government security detail, provided to all the presidential candidates.

Mousavi promised economic reform, freedom of expression and a campaign against economic corruption.

He also pledged to review laws that discriminate against women, remove the ban on privately owned television stations and curb the power of the supreme leader by taking control of security forces.