TEHRAN, Iran — Iran's powerful Guardian Council today announced it was ready to recount specific ballot boxes in last week's disputed presidential elections, another twist in an election that has touched off massive protests.

State television quoted spokesman Abbas Ali Kadkhodaei as saying that the recount would be limited to voting sites where candidates claim irregularities took place.

The results from last Friday's election showing a landslide victory for President Mahmoud Ahmadinejad sparked Tehran's worst violence in 10 years — including seven reportedly killed yesterday during clashes.

Supporters of reformist candidate Mir Hossein Mousavi claim the vote was rigged to re-elect the hardline president.

The 12-member Guardian Council includes clerics and experts in Islamic law. Its role includes certifying election results, and it is closely allied to Iran's supreme leader, Ayatollah Ali Khamenei.

It serves as a constitutional watchdog and vets candidates running in elections. It must certify ballot results and also has the apparent authority to nullify an election.

Iran state radio reported today that clashes in the Iranian capital the previous day left seven people dead during an "unauthorized gathering" at a mass rally over alleged election fraud — the first official confirmation of deaths linked to the wave of protests and street battles after disputed elections last week.

The report said the deaths occurred after protesters "tried to attack a military location." It gave no further details, but it was a clear reference to crowds that came under gunfire yesterday after trying to storm a compound for volunteer militia linked to Iran's powerful Revolutionary Guard.

The shootings came at the end of a huge rally by opponents of Ahmadinejad claiming widespread fraud in Friday's voting.

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

The protest movement has shown no signs of easing — with another reported rally planned for today.

In a message posted on his Web site, Mousavi said he will not attend the rally and asked his supporters "not fall in the trap of street riots" and exercise self-restraint."

Ahmadinejad, meanwhile, arrived in Russia today to attend a regional security summit, after having postponed the trip for one day.

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.

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!"

A Web site run by Iran's former reformist vice president, Mohammad Ali Abtahi, said he had been arrested by security officers, but provided no further details.

In the U.S., 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 followed hours of tension exacerbated by the government's blocking of text-messaging services and Web sites that protesters have used to organize street rallies. Sunday night, authorities shut off Tehran's cell phone system, and Mousa- vi'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 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 personally would take part to "calm the situation down."

"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 that "by the end of the week, Ahmadinejad will be gone."

When police did not materialize, the avenue between the two central squares quickly filled with people. "I'll fight, I'll die, but I'll get my vote back," a group of young men shouted.

Climbing over fences, surging down Azadi ("Freedom") Street, the march became a sea of people eight across and three to five miles long, numbering in the hundreds of thousands and, by some unofficial estimates, over 1 million. The crowds grew ecstatic when news spread that Mousavi and Mehdi Karroubi, another opposition presidential candidate, had arrived.

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 clerics, political leaders and military officers who have supported Ahmadinejad.