TEHRAN, June 15 -- Iran's supreme leader on Monday ordered a probe of Friday's presidential election, as protests over alleged voter fraud continued for a third straight day. But since Ayatollah Ali Khamenei has already congratulated President Mahmoud Ahmadinejad on his apparent reelection, some observers dismissed the investigation as an attempt to diffuse the anti-government demonstrations.

Pro-reform candidate Mir Hossein Mousavi and another candidate, Mahdi Karroubi, were planning to attend a large rally at Revolutionary Square Monday afternoon and to appeal for calm, aides said.

Mousavi is the leader of an opposition movement whose supporters appeared to turn out in force in Iran's urban areas to vote for a change in leadership. But hours after the polls closed, the Iranian government announced Ahmadinejad had been re-elected by a landslide.

Protests ensued, and Mousavi asked Iran's influential Guardian Council to declare the election null and void because of fraud and irregularities.

On Sunday, Mousavi met with Khamenei, a Shiite cleric who holds ultimate political authority in Iran. Ali Reza Adeli, a top aide to Mousavi, said Khamenei told Mousavi to use legal avenues to question the election outcome. The two leaders then jointly appealed for calm, Adeli said.

Khamenei later asked the Guardian Council to examine the election and the allegations of wrongdoing, and issue a report within seven to 10 days. The council is a 12-member commission that must validate the election before an official winner can be declared. Council members are appointed by Khamenei and the head of the judiciary. The council vets candidates and has the power to veto laws deemed inconsistent with Islam.

Although it has not yet acted to affirm the election results, state media have pronounced Ahmadinejad the victor, and Khamenei issued a statement congratulating him -- making a reversal extremely unlikely, according to many here.

Mousavi complained to the council about interference from security forces and the use of irregular procedures in the election. In asking the Guardian Council to nullify the election, Mousavi wrote on his Web site, "I believe this to be the only way to return the general trust and support of the people for the government." The site is now blocked in Iran.

Riots erupted on Sunday for a second day, with gunshots heard in several locations in Tehran and unrest in the Caspian Sea city of Rasht and the central Iranian city of Shiraz, witnesses said in phone interviews. On Sunday, Ahmadinejad led a victory rally near Vali-e Asr square attended by tens of thousands of people waving Iranian flags, which his campaign adopted as its symbol. At an earlier news conference, he declared the Iranian election a "true manifestation of people's right to decide their own destiny."

"Some . . . say the vote is disrupted, there has been a fraud," he said at the rally. "Where are the irregularities in the election?"

Young members of Iran's voluntary paramilitary force, the Basij, carried batons as they roamed the streets in groups on motorcycles, many wearing black helmets and green vests. Ahmadinejad said the demonstrators were "a few weeds that are making problems." To cheers, he said there would be no more place for them in Iran.

On his Web site, Mousavi wrote that "those who with great violations have issued results unbelievable for the Iranian nation are now trying to stabilize those results and start a new era in the history of our nation." He added that the protests over the results were not about him personally. "They are because of worries over the new method of political life which is being forced onto our country," he wrote.

During the campaign, Mousavi recalled, he warned many times of what he called illegal behavior by the Ahmadinejad government. "I stressed that such a method might result in despotism and dictatorship, and today our nation is at a point that it sees this fate looming," he said.

He urged the authorities to issue a permit for "massive" demonstrations in Iranian cities, writing that this would be the best way to stop the riots. "People will have a chance to show their protest and opposition to the method of the holding of the election and its result," Mousavi wrote, urging his supporters to continue using the color green, his campaign's signature color, in their protests. Cellphone service was cut for the second day in a row Sunday in an effort to quell riots.

Mousavi urged police forces not to use violence against protesters: "These people have come on the scene to acquire their rights and your rights and are your brothers and sisters. The power of our police and military forces has always been in their unity with the people, and it will remain so in the future." Many people have been severely beaten by special riot police and members of the Basij.

For the second night, fierce clashes broke out in Tehran and a growing number of other cities. Official security forces as well as members of the Basij attacked demonstrators and bystanders. Cars were burned in West Tehran, and shots were heard in the northwestern parts of the city. Iran's judiciary has said it will bring demonstrators swiftly to court.

Around 9 p.m. Sunday, people in many neighborhoods went to rooftops and balconies and chanted "God is great" in support of Mousavi. The rallying cry is the same one protesters used in the weeks leading up to the 1979 revolution that ousted Shah Mohammad Reza Pahlavi.

The Fars News Agency, which is close to the government, reported the arrest of dozens of political dissidents in Tehran, most of them supporters of Mousavi. Witnesses reported that students had broken out of a university that was surrounded by security forces, a student named Keyvan, who would not give his family name, said in a phone interview. In Rasht there were reports of casualties, Amir, an engineer, said in a phone interview.

During a news conference earlier Sunday, Ahmadinejad lauded the "epic" election. "This election was so free that you could say it was complete freedom," he said. But he added that the time had come to move on. "The election is gone and done. It is time for friendship, coalition and building the country."

When asked about protests and complaints, Ahmadinejad said that it was important to ask the opinions of "true Iranians" on the election. "Like the people you meet at my rallies," he said. He described the protesters as soccer hooligans who were disappointed that their team lost the match. "This is not important," he said. "We have full freedom in Iran."

His supporters showed up by the tens of thousands at a central Tehran square. Boys wearing tight T-shirts and women in traditional head-to-toe black chadors held up Iranian flags. "We are here to support our president," said Massoumeh Nazemi, who was sitting in the grass with her husband, Ali, and their daughter, Nargess, as Ahmadinejad spoke in the distance. "The police and security forces must deal very harshly with the demonstrators," Nazemi said. "They can't accept the victory of the Iranian people. They are hypocrites," she said, using a term commonly directed at those perceived to be enemies of the Islamic revolution.

"To those who say I create problems, I want to say that I am only a small drop in the ocean of the Iranian nation," Ahmadinejad declared, his voice amplified by loudspeakers. "You are against the nation," he said to his opponents. "There is no other choice than to surrender."

Crowds of mostly lower-class families cheered as Ahmadinejad again spoke of a corrupt Iranian ruling class that he said he was determined to bring to justice. "You think you are of the elite? That you are above the people?" he said, referring to a group of 200 people who he said were forcing themselves upon Iran. He again mentioned the children of prominent clerics, saying they were corrupt. "The society must be purified of these people," he said.

Ahmadinejad asked his supporters to be patient, explaining that such a purge would not happen overnight. "They will try to stop me," he said, "but I will expose them to the great nation of Iran."