Syrian security forces have shot and killed at least 15 people as they conducted raids and fired on protesters who flooded streets after Friday prayers.
Syrian activists say most of the killings took place in the northwestern region of Jabal al-Zawiya, in the southern province of Deraa and the central province of Hama, where at least five people were killed.
The London-based Syrian Observatory for Human Rights says security forces also killed two people in the central city of Homs as crowds of protesters clashed with Syrian forces.
The violence follows United Nations Secretary-General Ban Ki-moon's call for a "coherent" international response to Syria's violent crackdown against dissent, as the uprisings entered their seventh month.
Thursday Ban accused Syrian President Bashar al-Assad of "escalating violence and repression" and ignoring appeals to stop state-sponsored violence. The U.N. chief said "enough is enough" regarding Mr. Assad's broken promises of reform.
Also Thursday, Syrian opposition activists announced in Istanbul, Turkey, the formation of a council to present a united front against Assad. They said the council includes 140 members with half of them living in Syria.
The United States expressed its support, but noted the need for coordination as the group faces many challenges.
U.S. officials urged Americans to immediately leave Syria on Thursday, repeating their warning issued last month. Since then, Washington has imposed additional sanctions on the Syrian government.
The United Nations estimates that 2,600 people have been killed in Syria's six-month uprising against Assad's 11-year autocratic rule. The Syrian government said Monday about 1,400 have been killed, half of them security personnel.
Some information for this report was provided by AP and AFP.