Syrian security forces have used live ammunition on protesters calling for President Bashar al-Assad's ouster.

Activists and witnesses say forces opened fire and killed at least 15 people on Friday when thousands of anti-government demonstrators rallied in streets across the country.

They say protesters were killed in areas that include the cities of Hama, Homs and Aleppo, as well as the suburbs of Damascus.

Earlier Friday, government forces began carrying out raids in anticipation of the protests. Rights groups and activists say a woman was killed when tanks and soldiers launched an assault on Khan Sheikhon in the northern province of Idlib. Also, a man was shot and killed after fleeing from troops in the Damascus suburb of Saqba.

The Syrian government has defended its crackdown on dissent, saying it is combating armed gangs and terrorists. On Friday, state-run news reports said “armed groups” with “snipers” had opened fire “randomly” in a Damascus suburb and two other areas, killing three law enforcement officers and two civilians.

Mr. Assad has been facing growing international condemnation for the violence.

On Friday, U.S. Secretary of State Hillary Clinton urged countries to stop trading with Syria. She called on them to stop buying products such as oil and gas and also urged countries that have been supporting Mr. Assad's government to “get on the right side of history.”

Meanwhile, Turkish President Abdullah Gul called on Mr. Assad to implement reforms before it is too “late.” Turkish media reports relayed Mr. Gul's comments on Friday.

Rights groups say more than 1,700 people have been killed in Syria's crackdown.

Details of events in Syria are difficult to confirm because the government allows very few foreign news reporters into the country and restricts their movements.