The New York Times quoted human rights activists and residents as saying at least 15 people were killed during the protests by tens of thousands of Syrians despite government crackdowns this week in Hama and Deir al-Zour.
The violence came as diplomatic pressure increased on the Assad government to implement reforms.
"Today they were successful in dispersing the crowds by force," Saleh al-Hamawi, a Hama activist, told the Times by telephone. "But they failed to put fear in our hearts, which they are betting on."
Another man, who gave his name only as Hozan, said Assad's troops changed tactics and attacked worshipers as they left their mosques.
Hamawi said buses packed with soldiers and police surrounded every mosque in Hama.
Crowds chanted, "We will not kneel."
Activists said demonstrations were smaller than on previous Fridays but occurred in spite of the crackdown.
Armed militiamen known as shabiha, loyal to the government, burned and looted houses and shops. Earlier, mosques were closed and calls for prayer banned, Britain's The Guardian reported.
Forces were also reported in Nawa in the south, defined as the city in which the biblical figure Job lived and the burial place of Shem, son of Noah.
At least 10 people were reported killed in the crackdown Thursday and more than 400 were reported killed across the country since Assad launched his unyielding offensive July 31 on the eve of the Muslim holy month of Ramadan, which ends Aug. 29.
More than 2,000 people have been killed since the uprising began, rights advocates said.
U.S. Secretary of State Hillary Clinton Thursday urged other nations to increase pressure on Assad, suggesting in an interview with CBS News China and India impose energy sanctions and Russia stop selling arms to Syria.
Asked why Washington had not yet called for Assad to step down, Clinton said the administration first wanted other nations to add their condemnation.
"It's important that it's not just the American voice," she said. "And we want to make sure those voices are coming from around the world."
The White House is poised to call on Assad to give up power, administration officials said Wednesday.
Forty-one former members of the ruling Baath Party called this week for an end to the crackdown, warning the country would face "catastrophic results" otherwise.