Mkhitar Khachatryan, Associated Press
Opposition supporters wave an Armenian flag during a protest rally in Yerevan, Armenia. About 15,000 demonstrators marched through Yerevan to protest the results of the Feb. 19 election.

YEREVAN, Armenia — Armenia's president imposed a state of emergency Saturday after police used tear gas and fired shots into the air to disperse demonstrators protesting alleged fraud in last month's presidential election.

The announcement from the office of President Robert Kocharian came shortly after police broke up the rally of about 15,000 demonstrators. Earlier, police used batons to remove hundreds of protesters who had camped for more than a week in tents in a square near the city mayor's office.

Tensions remained high into the night. Groups of angry young people were prowling the streets of the capital, calling for others to join them, city residents told The Associated Press. An AP reporter saw cars overturned, and the police department said some of its cars had been set on fire. Looters hit stores and kiosks, police said.

"What's going on now is not a political process. It has gone over the edge," Kocharian said in a late-night news conference. "I appeal to the people of Armenia to show restraint and understanding."

Kocharian's office said the state of emergency would remain in effect for 20 days. The measure imposes severe restrictions, including banning all mass gatherings and ordering that news media reports on domestic political matters include only official information.

The order also says that police have the right to restrict movement and to search private and public vehicles.

Witnesses said they had seen people injured in the later rally, but there was no immediate official information on casualties. Kocharian said eight police officers were injured.

Kocharian says some of the demonstrators were armed and that police said they had been shot at.

The Armenian Health Ministry said 10 people were hospitalized from the earlier clash but did not say how severe their injuries were nor how many of the injured were police.

The opposition has protested the results of the Feb. 19 presidential election results and tried to force a new vote. Rallies daily have drawn tens of thousands of people; a few hundred remained at the square each night in tents.

Officials say Prime Minister Serge Sarkisian won the vote outright. But supporters of opposition candidate Levon Ter-Petrosian have rejected the results, alleging fraud.

Sarkisian is a close colleague of Kocharian, who is stepping down because the constitution does not permit him to seek a third term. Opponents allege the government manipulated the vote count, pressured people to vote for Sarkisian and pressured the news media into skewing coverage in favor of him.

The observer mission from the Organization for Security and Cooperation in Europe said there were concerns about the vote count, but it issued a generally positive assessment.

Sustained unrest in Armenia could undermine stability in the volatile Caucasus region. Armenia borders Turkey, Iran, Georgia and Azerbaijan — countries important for producing or shipping oil and gas to Western customers.

"At this important stage in Armenia's development, everything should be done to avoid any escalation of tension," said Finnish foreign minister Ilkka Kanerva, the chairman-in-office of the Organization for Security and Cooperation in Europe.

Ter-Petrosian, who was president of Armenia in the first painful years of independence from the Soviet Union, appealed to the Constitutional Court on Friday to overturn the results.

Security police on Saturday prevented Tar-Petrosian from leaving his residence, but he told reporters that he was not under formal house arrest.

"If he is accused of committing a crime, he should be properly charged and prosecuted in a court of law like anyone else. In a democracy you cannot arbitrarily detain political opponents," said Terry Davis, secretary-general of the Council of Europe, the continent's main human rights body.