There was a loud explosion, and a column of black smoke rose over Damascus, an Assad stronghold that until the past few days had seemed largely beyond the reach of rebels.
Dozens of wrecked cars were strewn over a car park used by lawyers and judges. The Syrian flag few at half-mast on the building, but there was no word on casualties.
Meanwhile, tension grew on Syria's northern borders as Turkish military convoys moved towards the Syrian frontier, reacting to Syria's shooting down of a Turkish warplane over the Mediterranean on Friday.
The build-up of defences coincided with a general escalation of violence in Syria itself and a growing sense of urgency in Western and Arab-backed diplomatic efforts to forge a unity government and end 16 months of bloodshed.
Prime Minister Tayyip Erdogan, speaking after Friday's incident, ordered his troops to treat any Syrian military element approaching the border as a military target. He gave no details, but this could cover Syrian forces pursuing rebels towards the border, or patrolling helicopters and warplanes. Syria said at the weekend it had killed several "terrorists" infiltrating from Turkey.
David Hartwell, Middle East analyst at IHS Jane's, called the Turkish action a "pragmatic, rational response" after the shooting down of the Turkish aircraft, which Syria insists was flying low and fast in Syrian air space. "Damascus has been warned once. I doubt there will be a second warning," Mr Hartwell said.
Turkey, in the forefront of Western efforts to remove Mr Assad from power, hosts more than 33,000 Syrian refugees as well as units of the rebel Free Syria Army (FSA) that is fighting to overthrow Mr Assad.
"I can confirm there are troops being deployed along the border in Hatay province. Turkey is taking precautions after its jet was shot down," a Turkish official said on condition of anonymity.
Members of the FSA, talking close to the border, said they did not believe the Turkish deployments were on a large scale or aimed at any cross-border intervention.
Turkey has in the past spoken of opening a humanitarian corridor on Syrian soil, if the refugee flow grew unmanageable or if the violence and killing became intolerable. Wary of igniting a regional sectarian conflagration, it has always insisted this would be possible only with United Nations backing. Western and Arab-backed efforts to forge a joint diplomatic approach with Russia have so far failed.
A senior opposition official said Syrian opposition groups would reject a political transition plan proposed by UN peace envoy Kofi Annan unless it explicitly required Mr Assad to step down before a unity government is formed.
Mr Annan's transition proposal is one of the main topics that Russia, the other four permanent UN Security Council members and key players in the Middle East will discuss at a meeting in Geneva tomorrow, according to UN diplomats.
Diplomatic sources at the UN said the plan Mr Annan will now pitch aims to start the political process without waiting for a ceasefire.