The counterdemonstrations were the opposition's first major show of force on the streets of Tehran since mid-September that coincided with state-sponsored protests against Israel.

Witnesses told the Associated Press that security forces — mainly paramilitary units from the elite Revolutionary Guard — swept through an opposition counterdemonstration in the city center, clubbing, kicking and slapping protesters.

There were no immediate reports on injuries or arrests in the clashes in Haft-e-Tir Square — about half a mile from the annual anti-American gathering outside the former U.S. Embassy.

The witnesses spoke on condition of anonymity because of fear of reprisals from authorities. Media restrictions imposed after the disputed presidential elections in June limit journalists to covering state media and government-approved events, such as the rally outside the former embassy.

The state-run Islamic Republic News Agency reported that police also used tear gas to disperse protesters in other parts of the city.

Authorities appeared determined to avoid opposition rallies overshadowing the anniversary of the embassy takeover. They had warned protesters days in advance against attempts to disrupt or overshadow the annual gathering outside the former embassy, which was stormed by militants in 1979 in the turbulent months after the Islamic Revolution. Fifty-two Americans were held hostage for 444 days.

Security forces fanned out around Tehran at daybreak on Wednesday after opposition leaders refused to call off their appeals for counterdemonstrations.

Volunteer militiamen linked to the Revolutionary Guard patrolled the streets on motorcycles — a familiar sight during the unrest following the controversial June election.

Many marchers Wednesday wore green scarves or wristbands that symbolized the campaign of Mir Housein Mousavi, who claims President Mahmoud Ahmadinejad stole the election through fraud. Mousavi and his allies, including former president Mohammad Khatami, appeared to encourage opposition protesters to return to the streets.

The full extent of the opposition marches was not immediately clear. Hundreds were seen in Haft-e-Tir Square, some chanting "Death to the Dictator."

Other witnesses — also speaking on condition of anonymity — said about 2,000 students at Tehran University faced off against security forces, but there were no immediate reports of violence.

Thousands gathered outside the former embassy, waving anti-American banners and signs praising the Islamic Revolution.

The main speaker, hard-line lawmaker Gholam Ali Haddad Adel, denounced the United States as the main enemy of Iran. He did not mention the talks with the West, including the United States, on Iran's nuclear program.

In Washington, President Obama noted the anniversary of the takeover of the U.S. Embassy and urged the two countries to move beyond the "path of sustained suspicion, mistrust and confrontation."

The hostage crisis "deeply affected the lives of courageous Americans who were unjustly held hostage, and we owe these Americans and their families our gratitude for their extraordinary service and sacrifice," Obama said in a statement.