The white-turbaned cleric is an unlikely enemy of the Iranian state. He was a confidant of the Islamic Republic's founder, Ayatollah Ruhollah Khomeini, and served seven years as speaker of parliament.
But at 72, in the wake of Iran's disputed presidential election, Mehdi Karroubi has become the fiery heart of a protest movement that has shaken the republic's foundations.
"I feel I am obliged to defend the rights of people," Karroubi said Monday during a rare interview with a Western news organization at his sparse north Tehran office. "I want it to be remembered in the future by coming generations that somebody someday from the clerical establishment stood up for his stances and principles to defend the people."
On Tuesday, authorities stormed his party's headquarters in west Tehran. They seized documents, computers and photographs and arrested Mohammad Davari, editor of his website, a party spokesman said. They also arrested Ali-Reza Beheshti, the top aide to Karroubi's ally Mir-Hossein Mousavi, reformist websites reported.
Karroubi's popular daily newspaper was shut down weeks ago. Hard-line commanders of the Revolutionary Guard and President Mahmoud Ahmadinejad have repeatedly called for his arrest.
But Karroubi has continued to defy authorities, calling for opposition supporters to join in street rallies Sept. 18 during Quds Day celebrations, an annual march in support of Palestinians and against Israel.
Karroubi, along with Mousavi, lost to Ahmadinejad in a June 12 presidential election marred by opposition claims of widespread vote fraud. He has been an outspoken critic of the ensuing crackdown on dissent. He shattered taboos by collecting and publicizing explosive allegations that prison guards raped detained protesters.
In the interview with The Times, he offered a rare inside view of a nascent Iranian protest movement, which remains under heavy surveillance by security forces.
"Political changes can come in two forms," he said. "The change we are calling for is change within the system and constitution, the observation of citizenship rights."
He was not specific about the opposition's strategy, but sketched out goals for the coming months: loosened news media restrictions, freedom of assembly, an end to trials of opposition figures and revised laws to prevent the hard-line Guardian Council from having the final say on elections.
The movement has few material or financial resources but morale is high. Karroubi said he didn't fear arrest or bodily injury. He has the same team of bodyguards he's had for years.
"I won't go underground," he said. "I act publicly and openly. Even if I am arrested and jailed and released, I will go back to open activities."
The protests that rocked the nation after the election were essential, he said, pressuring the government to act and demonstrating the extent of discontent.
"I know all the political factions, the left and right ones," he said. "Even if they muster up all their followers, they cannot bring out one-fifth of the people we have seen in the streets."
He ruled out as unwise more extreme actions such as general strikes.
"Common people would suffer at the end of the day," he said. "Our disputes are not so deep. It is a dispute between members of a family. So we do not need that scale of protest."
The large-scale demonstrations have subsided largely because of club-wielding, plainclothes Basiji militiamen unleashed against protesters. Karroubi decried what he described as abuses committed by the force, which fought during Iran's eight-year war against Iraq in the 1980s.
"Today's Basiji are different," he said. "They are not those who defended the country. Young kids are recruited into today's Basiji and are doing partisan political activities."
Still, he said that the organization, now under the command of the Revolutionary Guard, should not be demonized. "We should try to put the Basiji back on its original track."
Though he welcomed Iranian expatriates' involvement in their homeland's affairs, he said foreign governments should not interfere. But he said Iranian authorities had only themselves to blame for creating a situation that the West could exploit.
"Ordinary people want their votes to be counted," he said. "Certainly the world heeded their protests and tried to fan the fire. Honestly, if the U.S. administration does something wrong, do we not highlight it and make a hue and cry? Our enemy does the same to us."
But he also said he did not believe international sanctions on Iran over its nuclear program will help its reform movement. Nor will loud, widespread foreign condemnation of Ahmadinejad.
"Generally, I do not agree with any outside pressure on any government, as at the end of the day the ordinary people will suffer," he said.
"If foreign governments want to help, they must just stop being hostile toward us."
Mostaghim is a special correspondent.