A crackdown on dissent continued, with more arrests of opposition figures reported, and the country's most powerful military force - the Revolutionary Guard - saying that Iranian Web sites and bloggers must remove any materials that "create tension" or face legal action.
Supreme Leader Ayatollah Ali Khamenei has told Mousavi to pursue his demands through the electoral system and called for Iranians to unite behind their Islamic government, an extraordinary appeal in response to tensions over the presidential vote. But Mousavi appears unwilling to back down, issuing on his Web site a call for a mass demonstration tomorrow.
Tens of thousands of protesters gathered in Tehran today. Many were silent, wearing green wristbands or headbands, and making victory signs. Witnesses said that one street leading to Haft-e Tir square was packed for several kilometers.
"We want a peaceful rally to protest the unhealthy trend of the election and realize our goal of annulling the results," Mousavi said.
He called for "a new presidential election that will not repeat the shameful fraud from the previous election."
Web sites associated with Mousavi and the reformists called for at least one rally later Wednesday but the opposition leader made no reference to the gathering in his official statement.
Mousavi and his supporters accuse the government of rigging the June 12 election to declare hard-line President Mahmoud Ahmadinejad the overwhelming winner. Their street protests, paired with dissent from powerful clerical and political figures, have presented one of the gravest threats to Iran's complex blend of democracy and religious authority since the system emerged from the 1979 Islamic Revolution.
Blogs and Web sites such as Facebook and Twitter have been vital conduits for Iranians to inform the world about protests and violence.
The Web became more essential after the government barred foreign media Tuesday from leaving their offices to report on demonstrations on the streets of Tehran.
Mousavi condemned the government for blocking Web sites, saying the government did not tolerate the voice of the opposition.
The violence has left at least seven people dead, according to Iran's state media, although videos and photos posted by people inside Iran show scenes of violence that have not been reported through official channels. The new media restrictions make it virtually impossible to independently verify much of the information, which includes dramatic images of street clashes and wounded demonstrators.
Much of the imagery has been posted anonymously. In other cases, those who have posted have declined to be identified due to fear of government retaliation, or cannot be reached due to government restrictions on the Internet and mobile phones.
The Revolutionary Guard, an elite military force answering to Khamenei, said through the state news service that its investigators have taken action against "deviant news sites" that encouraged public disturbance and street riots. The Guard is a separate military with enormous domestic influence and control of Iran's most important defense programs. It is one of the key sources of power for the ruling establishment.
The statement alleged that dissident Web sites were backed by Canadian, US and British interests, a frequent charge levied by Iranian hard-liners against their opposition.
"Legal action will be very strong and call on them to remove such materials," it said.
Meanwhile, election tensions appeared to be spreading further into the Iranian political and religious classes.
The semiofficial ISNA news agency and the private ILNA news agency reported that scuffles broke out between two legislators — one a reformist and the other a hard-liner — in an open session of parliament after they argued about the vote results.
The agencies said hard-liner Ruhollah Jani Abbaspour attacked reformer Amir Taherkhani after a parliamentary committee probing the protests met Mousavi and the speaker of parliament gave a report on the probe.
Iran's most senior dissident cleric, Grand Ayatollah Hossein Ali Montazeri, said widespread vote fraud had undermined the legitimacy of the ruling Islamic system and that "no sound mind" would accept the results.
"A government that is based on intervening in (people's) vote has no political or religious legitimacy," said Montazeri, who had once been set to succeed Ayatollah Ruhollah Khomeini as supreme leader until he was ousted because of criticisms of the revolution.
The US-based International Campaign for Human Rights in Iran said that several dozen noted figures associated with the reform movement have been arrested, among them politicians, intellectuals, activists and journalists.
Tehran-based analyst Saeed Leilaz, who is often quoted by Western media, was arrested Wednesday by plainclothes security officers who came to his home, said his wife, Sepehrnaz Panahi.
At least 10 Iranian journalists have been arrested since the election, Reporters Without Borders said.
A Web site run by former Vice President Mohammad Ali Abtahi said the reformist had been arrested.
Prominent reformer Saeed Hajjarian has also been detained, Hajjarian's wife, Vajiheh Masousi, told The Associated Press. Hajjarian is a close aide to former President Mohammad Khatami.
In an attempt to placate the opposition, the main electoral authority has said it was prepared to conduct a limited recount of ballots at sites where candidates claim irregularities.
The recount would be overseen by the Guardian Council, an unelected body of 12 clerics and Islamic law experts close to Khamenei.
Mousavi charges the Guardian Council is not neutral and has already indicated it supports Ahmadinejad. He and the two other candidates who ran against Ahmadinejad are calling for an independent investigation.
His representative, reformist cleric Ali Akbar Mohtashamipour, said after a meeting with the council Tuesday the number of votes in counted in 70 districts was higher than the population in those districts. He also said many polling stations were closed sooner than scheduled on election night while people were still lining up.
On Tuesday, the government organized a large rally in Tehran to show it too can bring supporters into the streets. Speakers urged Iranians to accept the results showing Ahmadinejad was re-elected in a landslide.
The appeal for unity failed to calm passions, and a large column of Mousavi supporters marched peacefully in north Tehran, according to amateur video.
A witness told the AP that the pro-Mousavi rally stretched more than a mile (1.5 kilometers) along Vali Asr Avenue, from Vanak Square to the headquarters of Iranian state television.
Security forces did not interfere, the witness said, speaking on condition of anonymity for fear of government reprisal.
Ahmadinejad, who has dismissed the unrest as little more than "passions after a soccer match," attended a summit in Russia that was delayed a day by the unrest in Tehran. That allowed him to project an image as Iran's rightful president, welcomed by other world leaders.
In Washington, President Barack Obama expressed "deep concerns" about the legitimacy of the election and post-voting crackdowns.
