DUBAI—Syrians took to the streets in cities across the country in defiance of the government's lethal bid to clamp down on antiregime protests, with demonstrators appearing to gain support from tribal members and some army troops.

Syrians in several cities marched after Islamic prayers in what activists called a "Friday of Defiance." Activists and witnesses cited at least 27 deaths in four towns. Fifteen of the fatalities came in the central city of Homs, where witnesses say a bus full of security forces—identified by one...