TEHRAN, June 15 -- Hundreds of thousands of Iranians defied a ban by the Interior Ministry and marched through the capital on Monday in support of opposition presidential candidate Mir Hossein Mousavi, posing a rising challenge to the country's ruling clergy over the disputed election.
Though the afternoon march was peaceful and proceeded without police interference, at least one man was killed and several were wounded at nightfall, when members of the Basij, a volunteer militia allied with the government of President Mahmoud Ahmadinejad, fired from a rooftop into a crowd outside its local headquarters in downtown Tehran.
There were conflicting reports on whether the crowd had threatened to storm the building before the shooting, but the incident ended with angry protesters setting part of the structure and several motorcycles ablaze. Young Basij members on motorcycles have harassed and beaten protesters since Friday's election, in which the government says Ahmadinejad defeated Mousavi by 2 to 1.
The unrest, including scattered reports of violence elsewhere in the country, appears to have unsettled the country's unelected leadership of Islamic clergy. Hours before the march -- the largest unofficial demonstration in Iran since the 1979 Islamic revolution -- the country's supreme leader, Ayatollah Ali Khamenei, abruptly reversed course and promised an investigation into allegations of election fraud. Khamenei previously had blessed Ahmadinejad's victory.
"Offenses in elections are not out of the ordinary. We do not want to exaggerate and say that no violation has taken place," a spokesman for the investigating body, the Guardian Council, said on television. "No, humans are involved in this affair, and human beings are not free of faults."
"Bear with us," pleaded the spokesman, Abbas Ali Kadkhodai. "We will investigate and announce the result."
Mousavi, in his first public appearance since the election, told the cheering marchers that he did not put much faith in the independence of the council, a panel of 12 Islamic clerics and jurists selected by Khamenei, the head of the judiciary and Iran's parliament.
"I have appealed to the Guardian Council, but I'm not very optimistic about their judgment," he said. "Many of its members during the election were not impartial and supported the government candidate."
Mousavi added that he was "ready to pay any price" in his fight for an honest election. "I came here to invite everyone to defend their rights calmly," he said as thousands of supporters -- most of them wearing green, the signature color of his campaign -- chanted "Mousavi, we will help you!"
In Washington, President Obama said he was "deeply troubled by the violence" in Iran.
Because no international observers were allowed to monitor the fairness of the election, "I can't state definitively one way or another what happened with respect to the election," Obama said. "But what I can say is that there appears to be a sense on the part of people who were so hopeful and so engaged and so committed to democracy who now feel betrayed. And I think it's important that, moving forward, whatever investigations take place are done in a way that is not resulting in bloodshed and is not resulting in people being stifled in expressing their views."
The march through Tehran began in late afternoon, following hours of tension exacerbated by the government's continued blocking of text-messaging services and Web sites that protesters have used to organize street rallies. On Sunday night, authorities shut off Tehran's cellphone system, and Mousavi's wife, Zahra Rahnavard, told a gathering of university students that the march had been canceled because organizers expected Basij and riot police to use extreme force against the demonstrators.
Nevertheless, ardent Mousavi supporters continued Monday with plans to usurp the symbols of the 1979 Islamic revolution by parading from Revolution Square to Freedom Square, two focal points of the uprising that toppled the shah. In the early afternoon, Mousavi issued a statement saying he personally would take part to "calm the situation down."
Around 4 p.m., as the march got underway, dark clouds, highly unusual in this season, blocked the summer sun.
"Let's go, move!" organizers shouted as the first few rows of young men wearing green headbands and young women holding hands stepped forward, shouting in Farsi that "by the end of the week, Ahmadinejad will be gone."
When riot police did not materialize, the broad avenue between the two central squares quickly filled with people, including some families with children. "I'll fight, I'll die, but I'll get my vote back," a group of young men shouted, flashing the V-for-victory sign. Many wore green ribbons or wristbands.
"We are not weeds!" they chanted, a caustic reference to Ahmadinejad's statement over the weekend that the protesters were just "a few weeds that are making problems."