Assad, fighting the most serious challenge to his 11-year-rule, has sent troops and tanks into several cities in the last two weeks to try to bring to an end protests inspired by Arab revolts which toppled leaders in Egypt and Tunisia.

Protesters, who first called for reforms and greater freedoms, have hardened their demands with many chanting for the overthrow of the president who inherited the authoritarian powers of his father Hafez al-Assad, who died in 2000.

Wednesday was one of the bloodiest days apart from the main Friday protest days, when thousands use the platform of weekly Muslim prayers to demonstrate. Most of the violence occurred in the southern Deraa province, where unrest erupted on March 18.

Ammar Qurabi, head of the National Organization for Human Rights in Syria, said 13 people were killed in the town of Harra, about 60 km (40 miles) northwest of Deraa city.

Reuters