Iran's opposition announced a third day of street demonstrations Wednesday as the country's most powerful military force warned of a crackdown against online media in its first pronouncement on the deepening election crisis.
The Revolutionary Guard, an elite military force answering to Supreme Leader Ayatollah Ali Khamenei, said through the state news service that Iranian Web sites and bloggers must remove any materials that "create tension" or face legal action.
Meanwhile, Iran's Interior Ministry ordered an investigation into an attack on university students they say was carried out by Islamic militia and police.
Iran's English-language Press TV said the ministry had "called on Tehran's governor's office to identify those involved" in Sunday night's incident. It did not make clear whether the ministry itself would also look into it.
One student activist who declined to be named told Reuters on Tuesday that four students — three men and one woman — were killed during the violence.
Supporters of presidential candidate Mir Hossein Mousavi, who has accused the regime of vote-rigging, said they have called for another rally in Tehran on Friday at 1330 GMT, despite a ban on such gatherings.
Reformists also reported on Wednesday that another two prominent academics and journalists had been arrested by the authorities.
• YouTube Video May Show Gunman Killing Protesters in Iran
Iranian reformist Web sites as well as blogs and Western Web sites like Facebook and Twitter have been vital conduits for Iranians to inform the world about protests over the declaration of election victory for hardline President Mahmoud Ahmadinejad.
The Guards are a separate military with enormous domestic influence and control of Iran's most important defense programs. They are one of the key sources of power for a cleric-led establishment that has been pushed by the crisis into an extraordinary public defense of the Islamic ruling system.
The unelected supreme leader made a highly unusual appeal Tuesday in response to tensions, telling Iranians that all citizens should hold fast to their belief in the system despite disagreements over the election.
Despite official warnings and a ban on street demonstrations, some Web sites allied with opposition leader Mousavi said Wednesday that supporters should gather in a downtown square in the late afternoon.
The announcement raised the prospect of further clashes with security forces. The violence has left at least seven people dead, according to Iran's state media.
In an attempt to placate the opposition, the main electoral authority said Tuesday it was prepared to conduct a limited recount of ballots at sites where candidates claim irregularities.
But the announcement did not satisfy Mousavi, who has called the election an "astonishing charade" and demanded it be canceled and held again.
His representative, reformist cleric Ali Akbar Mohtashamipour, reiterated that demand after a meeting of the Guardian Council, an unelected body of 12 clerics and Islamic law experts close to the supreme leader and seen as supportive of Ahmadinejad. Mousavi and the two other candidates who ran against Ahmadinejad are calling for an independent investigation of voting irregularities.
Mousavi charges the Guardian Council is not neutral and has already indicated it supports Ahmadinejad.
"If the whole people become aware, avoid violent measures and continue their civil confrontation with that, they will win. No power can stand up to people's will," Mohtashamipour said. "I do not think that the Guardian Council will have the courage to stand against people."
On Tuesday afternoon, the government organized a large rally in Tehran to show it too can bring supporters into the streets. Thousands waved Iranian flags and pictures of the supreme leader, thrusting their fists into the air and cheering as speakers denounced "rioters" and urged Iranians to accept the results showing Ahmadinejad was re-elected in a landslide Friday.
"This nation will protect and defend its revolution in any way," Gholam Ali Haddad Adel, a prominent lawmaker and Ahmadinejad supporter, told the pro-government crowd in Vali Asr Square.
He called on Mousavi's supporters to accept the results and press their complaints through legal means.
"After all, in all elections there will be losers and winners, naturally," he said. "This should not cause a rift between the people."
The appeal for unity failed to calm passions, and a large column of Mousavi supporters — some of them with green headbands and their faces masked against tear gas or to hide their identities — marched peacefully along a central avenue in north Tehran, according to amateur video.
A witness told The Associated Press 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, and the protest lasted from about 5 p.m. to 8 p.m. Other witnesses told the AP that about 100 people continued the protest in front of state TV past 9:45 p.m. The witness spoke 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.
The U.S.-based International Campaign for Human Rights in Iran said that several dozen noted figures associated with Iran's reformist movement had been arrested Tuesday, among them politicians, intellectuals, activists and journalists.
At least 10 Iranian journalists have been arrested since the election, "and we are very worried about them, we don't know where they have been detained," Jean-Francois Julliard, secretary general of Reporters Without Borders told AP Television News in Paris. He added that some people who took pictures with cell phones also were arrested.
A Web site run by former Vice President Mohammad Ali Abtahi said the reformist had been arrested.
Saeed Hajjarian, a prominent reformer, also has been detained, Hajjarian's wife, Vajiheh Masousi, told the AP. Hajjarian is a close aide of former President Mohammad Khatami.
Iran's most senior dissident cleric said Tuesday that the ruling Islamic system had no political or religious legitimacy because of widespread vote fraud in Friday's presidential election.
Grand Ayatollah Hossein Ali Montazeri said "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 Khomeini as supreme leader until he was ousted because of criticisms of the revolution.
The Associated Press contributed to this report.
• Crisis in Iran Sparks Global Guerrilla Cyberwar
• Twitter Links Iran Protesters to Outside World
• How Iranians Get Around Web Censors