Two car bomb attacks on Syrian security sites in Damascus killed several civilians and military personnel, state television said, in blasts that witnesses said rocked the capital.
Initial footage of the blast showed piles of blackened rubble and medics wrapping bloodied bodies in blankets.
A Reuters cameraman was not allowed to enter the site, but video broadcast by the state news channel showed smoke rising from buildings with holes blasted through them, and the charred remains of the cars.
It said initial investigations indicated that al Qaeda was behind the attacks.
A witness who spoke to Reuters by telephone said two blasts shook the capital and the opposition Syrian Observatory for Human Rights said the blasts were followed by bursts of heavy gunfire.
The attacks came a day after the first wave of an Arab League observation mission landed in Damascus to prepare for an monitoring team that will check if Syria is implementing a peace plan to halt months of bloodshed.
Syrian security forces have launched a fierce crackdown on nine months of anti-government protests that has left 5,000 people dead, according to the United Nations.
Syria says it is fighting foreign-backed "terrorists" that have killed 2,000 members of its security forces.
In recent months, the peaceful pro-democracy movement that began in March has been overshadowed by pockets of armed insurgency that have begun to launch what they call defensive attacks against Syrian forces.
Friday's attacks hit a state security administration building and a local security branch, state television said.
Last month, a small blast was reported near a Syrian intelligence building in Damascus, but there was little damage.