Many in the huge crowd carried black candles and lit them as night fell. Others wore green wristbands and carried flowers in mourning as they filed into Imam Khomenei Square, a large plaza in the heart of the capital named for the founder of the Islamic Revolution, witnesses said.
Press TV, an English-language version of Iranian state television designed for foreigners, estimated the crowd at hundreds of thousands and said the people listened to a brief address from Mousavi, who called for calm and self-restraint.
A Mousavi Web site said that the crowed exceeded 1 million.
Independent witnesses said that, based on previous demonstrations at the site, the size of the crowd appeared to be in the hundreds of thousands. Foreign news organizations are barred from reporting on Tehran's streets.
The demonstrators had marched silently until they arrived at the square, where some chanted "Death to the dictator!" one witness said. Press TV showed protesters making V-for-victory gestures and holding pictures of Mousavi and signs that say "Where's our Vote?"
A participant told The Associated Press by telephone that the rally stretched for more than three miles (5 kilometers) through downtown Tehran from the square.
Photos posted online showed Mousavi talking through a handheld loudspeaker, dressed in a black suit and dark blue as he raised one hand to address the massive crowd.
The participant confirmed the authenticity of the images.
"When we started to walk, when there was a bridge, it was like a sea of people across the bridge," the participant said.
For three hours, a constant stream of people crossed the bridge, he said.
"I remember one old man talking about how the will of the people has started and no one can stop it," he added.
The participant and the witnesses spoke to The Associated Press on condition of anonymity for fear of government retaliation.
On their way home, some demonstrators held a candlelit gathering in front of Tehran University, where Mousavi supporters have accused pro-government militia of attacking students in dormitories.
On Monday, hundreds of thousands turned out in a huge procession that recalled the scale of protests during the 1979 Islamic Revolution. Seven demonstrators were shot and killed that day by pro-regime militia in the first confirmed deaths during the unrest.
After dark Thursday - as they have done on other nights this week - people went to their roofs and chanted, "Mir Hossein!" in support of Mousavi, and "God is great!"
Ahmadinejad released a largely conciliatory recorded statement on state TV, distancing himself from his past criticism of protesters, whom he has compared to angry soccer fans and "dust."
"I only addressed those who made riot, set fires and attacked people," the statement said. "Every single Iranian is valuable. The government is at everyone's service. We like everyone."
Iran's supreme leader, Ayatollah Ali Khamenei, has urged the people to pursue their allegations of election fraud within the limits of the cleric-led system. Mousavi and his followers have rejected compromise and pressed their demands for a new vote, flouting the will of a man endowed with virtually limitless powers under Iran's constitution.
Trying again to satisfy the protesters' demands, the main electoral authority invited Mousavi and two other candidates who opposed Ahmadinejad to a meeting. Iran's al-Alam Arabic television channel said the three candidates would meet with the Guardian Council on Saturday.
The unelected body of 12 clerics and Islamic law experts close to Khamenei has said it was prepared to conduct a limited recount of ballots at sites where candidates claim irregularities.
Mousavi, who has said he won the election, says the Guardian Council supports Ahmadinejad and has demanded an independent investigation, as well as a new election.
The council's spokesman, Abbasali Khadkhodaei, said Thursday that it received a total of 646 complaints from the three candidates who ran against Ahmadinejad in the June 12 election.
The council provided few other details, but the large number of complaints raised the possibility that even a limited recount could turn into a far larger and messier exercise than the government desires.
Presidential candidate Mahdi Karroubi said he didn't recognize Ahmadinejad as president, accusing his government of committing "treason."
"I consider Friday's election as illegitimate and don't accept the results announced," Karroubi said in a statement posted on his National Confidence Party's Web site.
The regime has blocked communication channels, such as Web sites and mobile phone networks, to make it more difficult for Mousavi supporters to organize protests. The mobile phone network in Tehran appeared to go down at the start of Thursday's demonstration, as it has intermittently since shortly after the election results were announced. Text messaging has been blocked almost constantly since Friday.
There have been widespread accusations of nighttime attacks on Mousavi supporters by pro-government militiamen, and protesters attacked a militia building after one rally, but both sides have been restrained, with uniformed police and other security forces standing by as protesters march calmly.
Monday's massive gathering was followed by three days of marches along main Tehran avenues, presenting one of the gravest threats to Iran's complex blend of democracy and religious authority since the system emerged out of the Islamic revolution that brought down Western-backed Shah Mohammad Reza Pahlavi.
The ruling clerics still command deep public support and are defended by Iran's most powerful military force - the Revolutionary Guard - as well as a vast network of militias.
But Mousavi's movement has forced Khamenei into the center of the escalating crisis, questioning his role as the final authority on all critical issues.
The wild card is former President Hashemi Rafsanjani, who heads the Assembly of Experts - a cleric-run body that is empowered to choose or dismiss Iran's supreme leader. Khamenei is Ayatollah Ruhollah Khomeini's successor, and the assembly has never used its power to remove Iran's highest authority.
Rafsanjani was a fierce critic of Ahmadinejad during the election, but has not publicly backed Mousavi. It is not known whether Mousavi has actively courted Rafsanjani's support or if they have held talks.
But Iranian TV has shown pictures of Faezeh Hashemi, Rafsanjani's daughter, speaking to hundreds of Mousavi supporters, carrying pictures of Khomeini.
A group of hard-line students rallied outside the Tehran prosecutor's office Thursday, accusing Rafsanjani's daughter and his son, Mahdi, of treason, state radio reported.
Protesters have focused on the results of the balloting rather than challenging the Islamic system of government. But a shift in anger toward Iran's non-elected theocracy would sharply change the stakes and become a showdown over the foundation of Iran's system of rule - the almost unlimited authority of the clerics at the top.
The Iranian government directly accused the United States of meddling in the deepening crisis. A statement by state-run Press TV blamed Washington for "intolerable" interference. The report, on Press TV, cited no evidence.
"Despite wide coverage of unrest, foreign media have not been able to provide any evidence on a single violation in the election process," state radio said.
State TV on Thursday broadcast the purported confession of a man accused of conspiring with U.S. forces in Iraq to bomb targets inside the country.
U.S. officials shrugged off the allegation of interference. President Barack Obama said he shared the world's "deep concerns" but it was "not productive, given the history of U.S.-Iranian relations, to be seen as meddling."
The two countries severed diplomatic relations after militants seized the U.S. Embassy in Tehran following the Islamic Revolution.
The government has blocked certain Web sites, such as BBC Farsi, Facebook, Twitter and several pro-Mousavi sites that are vital conduits for Iranians to tell the world about protests and violence. Many other sites, including Gmail and Yahoo, were unusually slow and rarely connect.
Mousavi has condemned the blocking of Web sites, saying the government did not tolerate the voice of the opposition.
In a statement, Google Inc.'s video-sharing site, YouTube, reiterated that its guidelines do allow clips depicting violence in Iran because of their journalistic merit. YouTube generally bans clips with graphic or gratuitous violence, but has made exceptions for video with educational, documentary of scientific value.
"The limitations being placed on mainstream media reporting from within Iran make it even more important that citizens in Iran be able to use YouTube to capture their experiences for the world to see," the company said. "Given the critical role these videos are playing in reporting this story to the world, we are doing our best to leave as many of them up as we can."
Iranian Press TV said Khamenei would lead the weekly prayers ceremony on Friday. There was no immediate word whether Ahmadinejad would attend, but attends the service whenever Khamenei gives it. Al-Alam said the three presidential candidates also confirmed they would attend.
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