DAMASCUS, Syria – A car bomb exploded Thursday near Syria’s ruling party headquarters in Damascus, killing at least 53 people and scattering mangled bodies among the blazing wreckage in one of the bloodiest days in the capital since the uprising began almost two years ago.

Elsewhere in the city, two bombs struck intelligence offices, killing 22, and mortar rounds hit the army’s central command, activists said.

Recent rebel advances in the Damascus suburbs, combined with the bombings and three straight days of mortar attacks, mark the most sustained challenge of the civil war for control of the seat of President Bashar Assad’s power.

Syrian state media said the car bombing near the Baath Party headquarters and the Russian Embassy was a suicide attack that killed 53 civilians and wounded more than 200, with children among the casualties. Anti-regime activists put the death toll at 61, which would make it the deadliest Damascus bombing of the revolt.

The violence has shattered the sense of normalcy the Syrian regime has tried to maintain in Damascus, a city that has largely been insulated from the bloodshed and destruction that has left other urban centers in ruins.

The rebels launched an offensive on Damascus in July after a stunning bombing on a high-level government crisis meeting that killed four top regime officials, including Assad’s brother-in-law and the defense minister. After that attack, rebel groups that had established footholds in the suburbs pushed in, battling government forces for more than a week before being routed and swept out.

Since then, government warplanes have pounded opposition strongholds on the outskirts, and rebels have managed only small incursions on the city’s southern and eastern sides.

But the recent bombings and mortar attacks suggest that instead of trying a major assault, rebel fighters are resorting to guerrilla tactics to loosen Assad’s grip on the heavily fortified capital.

The fighting in Damascus also follows a string of tactical victories in recent weeks for the rebels – capturing the nation’s largest hydroelectric dam and overtaking airbases in the northeast – that have contributed to the sense that the opposition may be gaining some momentum.

But Damascus is the ultimate prize in the civil war, and many view the battle for it the most probable endgame of a conflict that according to U.N. estimates has killed nearly 70,000 people.