Tehran, Iran (CNN) -- Protesters clashed with police at a central Tehran square on Wednesday while government supporters nearby marked the 30th anniversary of the U.S. Embassy capture with chants of "Death to America."
Scenes in the Iranian capital turned ugly yet again as riot police and pro-government Basij militia turned out in force to quash anti-government sentiment.
At least 2,000 opposition supporters, sternly warned by authorities to stay home, marched defiantly at Haft-e-Tir Square, witnesses said. Many held up their hands in a V sign. Others shouted "Allahu Akbar," or "God is great," a slogan of protest. Police blocked all roads leading to the square, prompting massive traffic jams.
Witnesses described helmet-clad security personnel beating demonstrators with batons and firing tear gas at Haft-e-Tir Square and in a neighborhood a few kilometers north. iReport: Iranians take to the streets
"I had never seen that many riot police and security personnel," a witness told CNN. "They were brought in by the busloads. As soon as crowds gathered somewhere, riot police were there within minutes."
Iranian reformists have chosen to hijack key anniversaries to protest the hardline government and Wednesday was the biggest annual anti-American observance of all.
On November 4, 1979, Islamic students stormed the U.S. embassy and held 52 Americans hostage for 444 days. Iran celebrates the embassy takeover as an official holiday. Tens of thousands showed up to hear anti-American speeches in front of the building that once housed the U.S. diplomatic corps.
Police quickly dispersed the crowds once the rallies were over.
But the opposition showed Wednesday that even after five months of government crackdown, people were still willing to take risks.
A disputed June 12 presidential election triggered Iran's most serious political crisis since the Islamic revolution topped the shah. Led by opposition candidate Mir Houssein Moussavi, thousands of Iranians protested what they believed was a rigged vote that returned hardline President Mahmoud Ahmadinejad to office for a second term.
In the election aftermath, the Iranian government arrested more than 1,000 people, accusing several reformists Moussavi and Mehdi Karrubi of spreading anti-government propaganda and fueling anger among the public.
But despite warnings from the government, Iran's reformists have largely refused to back down. They released the names of 72 protesters they say were killed in the unrest that followed the election -- more than double the government's official number.
In Washington, President Obama said the world continues to bear witness to the Iranian people's "calls for justice and their courageous pursuit of universal rights."
"Iran must choose," he said in a statement late Tuesday. "We have heard for thirty years what the Iranian government is against; the question, now, is what kind of future it is for.
"It is time for the Iranian government to decide whether it wants to focus on the past, or whether it will make the choices that will open the door to greater opportunity, prosperity, and justice for its people," he said.
Obama noted how the embassy takeover reshaped U.S.-Iranian relations.
"This event helped set the United States and Iran on a path of sustained suspicion, mistrust, and confrontation," he said. "I have made it clear that the United States of America wants to move beyond this past, and seeks a relationship with the Islamic Republic of Iran based upon mutual interests and mutual respect."
Obama said America has demonstrated willingness to work with Iran over nuclear issues by recognizing its right to peaceful nuclear power and accepting a recent proposal by the watchdog International Atomic Energy Agency.
"We have made clear that if Iran lives up to the obligations that every nation has, it will have a path to a more prosperous and productive relationship with the international community,' Obama said.
CNN's Reza Sayah and journalist Shirzad Bozorgmehr contributed to this report.