TEHRAN, Iran - Hundreds of thousands of Iranians defied a ban by the Interior Ministry and marched through the capital yesterday in support of opposition presidential candidate Mir Hossein Mousavi, posing a rising challenge to the country's ruling clergy over the disputed election.

Although the afternoon march was peaceful and proceeded without police interference, at least one man was killed and several wounded at nightfall, when members of the Basij, a volunteer militia allied with President Mahmoud Ahmadinejad's government, 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 a 2-1 ratio.

The unrest, including scattered reports of violence elsewhere in the country, has 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," a spokesman for the investigating body, the Guardian Council, said on television. "We do not want to exaggerate and say that no violation has taken place. No, humans are involved in this affair, and human beings are not free of faults."

Mousavi, in his first public appearance since the election, told 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.

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 election's fairness, "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, after 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 cell-phone system.

Nevertheless, ardent Mousavi supporters continued yesterday 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 would take part to "calm the situation down."

The march began about 4 p.m. 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 sign for victory.

Climbing over fences, surging down Freedom Street, the march became a sea of people eight across and three to five miles long, surely numbering hundreds of thousands and, by some unofficial estimates, more than one million. The crowds grew ecstatic when news spread that Mousavi and Mehdi Karroubi, another opposition presidential candidate, had arrived.

Protests also were reported in other cities. Police clashed with pro-Mousavi demonstrators in Esfahan and Mashhad, and authorities fired in the air to disperse protesters in Shiraz.

Besides ordering the investigation into allegations of election fraud, Khamenei also sought to calm the opposition by meeting with Mousavi before the march. The supreme leader told Mousavi that "enemies of the Islamic revolution" were behind the two nights of rioting and chaos on the streets of Tehran that followed the election. But "you are of a different kind from these people, and it is necessary to follow up things with composure and calm," Khamenei said, according to the official Islamic Republic News Agency.

Ahmadinejad, meanwhile, kept a low profile but stayed in Tehran, postponing a scheduled trip to Moscow.

It was unclear how easily Iran's leaders could alter the election result. Khamenei and some other leading clergy have already congratulated Ahmadinejad on his victory, and overturning the official results would not only embarrass the government but also might diminish the authority of a wide number of influential clerics, political leaders, and military officers who have supported Ahmadinejad since he won a surprise election victory in 2005.

Yet if Ahmadinejad stays on, his legitimacy may continue to be questioned. The head of Iran's parliament, Ali Larijani, also announced the formation yesterday of a committee to look into the election results.

He complained of "reports received by the parliament on attacks against the people," an apparent reference to the activities of the Basij and others over the weekend, when vigilantes attacked dormitories at Tehran University.