TEHRAN, Iran - Fiery chaos broke out in downtown Tehran yesterday as security forces blocked streets and used tear gas, water cannons, and batons to break up a demonstration against the reelection of President Mahmoud Ahmadinejad. Security forces were seen firing warning shots into the air, but there were also unconfirmed reports that several people were hit by gunfire.

President Obama, in his strongest comments to date on a political standoff that has paralyzed Iran for a week, urged the Iranian government "to stop all violent and unjust actions against its own people."

Opposition candidate Mir Hossein Mousavi, who asserts that he was cheated of victory in the June 12 election, said his supporters in the streets were "facing unrighteous liars."

Mousavi, in a statement posted on his campaign Web site, seemed to seek to avoid a direct confrontation with Ayatollah Ali Khamenei, Iran's supreme leader, who warned protesters of potential "bloodshed" if they continued mass street demonstrations. Mousavi said the way to restore calm on the streets was for the government to "not only allow for peaceful protests, but to encourage them."

Amid severe restrictions on news media reporting of the protests and conflicting accounts coming out of Tehran yesterday, some reports suggested Mousavi was taking a more confrontational stance. The Reuters news agency reported him as saying he was "ready for martyrdom" and vowing to continue his protest movement despite Khamenei's warning. But with foreign journalists prohibited from leaving their offices to witness the protests, those comments could not be independently verified.

The struggle on the streets of Iran continued to reverberate around the world yesterday as tens of thousands of Iranian exiles from across Europe and beyond gathered in a Paris suburb to cheer on the demonstrators in Tehran and demand an end to Iran's religion-based political system.

In Tehran, the street demonstrations were smaller than the massive protests that have jammed the capital for nearly a week. Khamenei's warning and a huge array of police on foot and motorcycles, as well as the pro-government Basij militia, clearly deterred some protesters. Nonetheless, thousands took to the streets, chanting slogans such as "Death to the dictator" and "Allahu akbar" ("God is great"), before police fired tear gas and water cannons to break up the gathering. News services estimated the crowd at about 3,000.

The number of casualties from running street clashes between security forces and protesters was not immediately clear. One witness said he saw three bodies being loaded into vans.

Residents of the area described firefights after protesters grabbed weapons from security forces. They also said a mosque was set on fire by people they described as "hooligans." Other witnesses said they saw people being shot.

The Associated Press reported that 50 to 60 protesters were seriously beaten by police and militiamen and taken to a hospital in central Tehran. Demonstrators could be seen dragging away comrades bloodied by baton strikes, the AP said.

Iran's official Press TV, an English-language version of state television, reported "sporadic clashes . . . between security forces and the protesters." The acting police chief, Brig. Gen. Ahmad-Reza Radan, said that the protests were illegal and that police would deal with them "firmly and with determination."

In a separate development, state-run news media reported that a suicide bomber blew himself up near a shrine to the late Ayatollah Ruhollah Khomeini, the father of Iran's 1979 Islamic revolution. State-run news media said the blast killed the bomber and injured three other people.

Riot police on motorcycles aimed to head off marchers as Shiite Muslim clerics who had joined the protesters tried to lead them to Revolution Square, witnesses said.

The confrontations came a day after Khamenei, Iran's ultimate political and religious authority, expressed his full support for the reelection of Ahmadinejad, rejecting allegations of vote fraud and declaring that foreign "enemies," including the United States, were behind the demonstrations.

By placing his seal of approval on the election's official results, Khamenei significantly raised the stakes for Iran's political opposition, which must now concede the election or be seen as challenging the supreme leader directly. So far, Mousavi and his supporters have questioned the election's validity but not the country's theocratic system of governance.

Ahmadinejad publicly thanked Khamenei for effectively endorsing his reelection. In a message to Khamenei carried by several local news agencies, the president said: "You wave the flag of honor over the head of our nation. You stood against the oppressors."

Mousavi, whose whereabouts yesterday were not immediately known, renewed his call to cancel the results of the election in an open letter published on his campaign Web site.