Syrian security forces killed at least four civilians on Friday in the central provinces of Homs and Hama, where anti-regime protests were set to take place, a rights watchdog said.

Three people died when troops opened fire on their vehicle near an intersection in Hama, and a fourth person was shot in the region of Homs, said the Syrian Observatory for Human Rights.

Opposition activists have called for protests across the country after the weekly Muslim prayers under the slogan "Our loyalty (to the revolution) is our salvation".