BEIRUT, Lebanon — Tens of thousands of Syrians in cities and towns around their country took to the streets on Friday after noon prayers shouting “We will not kneel” in a strong show of defiance against the government of President Bashar al-Assad, and at least 15 protesters were killed by security forces, human rights activists and residents said.

The demonstrations were smaller than those held in past weeks, but they were significant because they came after security forces took control this month of the country’s two most restive cities, Hama in central Syria and Deir al-Zour in the east, in a military operation that activists say left hundreds dead. The demonstrations on Friday were a clear sign the armed forces could not intimidate protesters into staying home.

Hundreds of security troops were seen converging around mosques in a number of Syrian cities and towns, often firing in the air in an attempt to prevent departing worshipers from forming crowds. Some mosques in Hama were even closed. In one protest on the outskirts of Damascus, the capital, members of the security forces outnumbered the demonstrators.

“Today they were successful in dispersing the crowds by force,” Saleh al-Hamawi, an activist from Hama, Syria’s fourth-largest city and a linchpin of the uprising, said by phone. “But they failed to put fear in our hearts, which they are betting on.”