TEHRAN, Iran - Opposition protesters returned to the streets of Tehran yesterday for the first time in nearly two months, clashing with security forces just blocks from a government rally to mark the 30th anniversary of the U.S. Embassy takeover.

The scenes of defiant chants, tear gas, and baton-wielding police recalled the unrest that followed the disputed presidential elections in June. But the latest marches drew far fewer demonstrators and suggested that the relentless pressure by authorities could be taking a toll on the opposition.

It also displayed the pinpoint counter strategy of opposition groups: staging rival marches during key state-backed events to gain maximum exposure as they try to reassert their voice. The contrasts were vivid on a day of major symbolic importance to the Iran's leadership.

People chanted "Death to America" and walked over U.S. flags outside Washington's former embassy. Meanwhile, hundreds of opposition marchers in nearby Haft-e-Tir Square denounced President Mahmoud Ahmadinejad with cries of "Death to the dictator" and trampled a poster of Iran's supreme leader, Ayatollah Ali Khamenei, witnesses said.

Marchers also called on President Obama to pick a side, as his administration pursues talks with the government. "Obama, Obama, you are either with them or with us," they chanted in Farsi in an amateur video clip widely circulated on the Internet.

In Washington, Obama called on the Iranian government to decide whether it wanted to focus on the past or the future. "Iran must choose," he said in a statement marking the anniversary of what he described as the unjust hostage-taking.

Several thousand protesters joined the marches. But authorities were ready with the same sweeping measures: dispatching paramilitary units to key locations and disrupting mobile phones, text messaging, and Internet access to frustrate protest organizers.

The main marches were quickly dispersed by security forces - including paramilitary forces and militiamen linked to the powerful Revolutionary Guard - who used clubs and tear gas, said witnesses, who spoke on condition of anonymity because of fear of reprisals from authorities.

Pro-reform Web sites said police fired into the air to try to clear the square, about half a mile from the former U.S. Embassy. The report could not immediately be independently verified. Groups of protesters were chased within a few blocks of the former U.S. Embassy as security forces fanned out through Tehran's center. Some demonstrators were injured and arrested, witnesses said, but there were no reports of serious injuries.

Yesterday's turnout was smaller than those during the summer's election protests, raising questions about the long-term stamina of the opposition after facing months of arrests and crackdowns.

Protesters still wear the green colors that symbolized the campaign of Mir Hossein Mousavi, who alleges Ahmadinejad stole the election from him through rigged ballots. But it has now expanded into a catchall movement for complaints that include the unlimited powers of the ruling clerics, Iran's sinking economy, and its international isolation.

Yesterday, Mousavi was prevented from leaving his office to attend the marches by militiamen on motorcycles, a pro-reform Web site said. And another leading opposition figure, Mahdi Karroubi, 72, fell to the ground after being overcome by tear gas, according to a posting by his son Hossein on Karroubi's Web site. Karroubi did not need medical attention, his son said.

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 Iranians in 1979 in the turbulent months after the Islamic Revolution. Fifty-two Americans were held hostage for 444 days in a crisis that began a three-decade diplomatic freeze between the two nations.

Outside the former embassy, thousands of people waved anti-American banners and signs praising the Islamic Revolution. As in past years, many were students bused in from outside Tehran.

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.