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.
Two other bombs struck intelligence offices, killing 22, 19 of them security officers. The Syrian Observatory for Human Rights, an anti-Assad group based in Britain, said three separate car bombs exploded near different security facilities in the northeastern Damascus neighborhood of Barzeh, followed by intense clashes between rebels and regime forces.
Recent rebel advances in the suburbs of the Syrian capital, 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 that the Syrian regime has desperately 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.
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 the ancient city as the most probable endgame of a conflict that according to U.N. estimates has killed nearly 70,000 people.
Thursday's car bomb hit a checkpoint on a bustling thoroughfare in the central Mazraa neighborhood between the Baath Party headquarters and the Russian Embassy. The force of the explosion shattered the balconies of apartment blocks along the tree-lined street and blew out the windows and doors of the party building.
Video of the blast site on Syrian state TV showed firefighters dousing a flaming car with hoses, while lifeless and dismembered bodies were tossed onto the grass of a nearby park. The state news agency, SANA, published photos showing a large crater in the middle of the rubble-strewn street and charred cars with blackened bodies inside.
There was no immediate claim of responsibility for the attack, but suspicion will likely fall upon one of the most extreme of Syria's myriad rebel factions, Jabhat al-Nusra.
The group, which the United States has designated a terrorist organization, has claimed past bombings on regime targets, including the double suicide blast outside an intelligence building in May that killed 55.
Such tactics have galvanized Assad's supporters and made many other Syrians distrustful of the rebel movement as a whole, most of whose fighters do not use such tactics.
The main opposition group, the Syrian National Coalition, condemned Thursday's bombing without accusing a specific group of carrying it out. It did, however, suggest that the regime allowed foreign terrorist groups to operate in Syria.
U.S. State Department spokeswoman Victoria Nuland condemned the "indiscriminate violence against civilians."
Russia's state-owned RIA Novosti news agency quoted a Russian Embassy official as saying its building had been damaged in the blast but no one was hurt.
State media also reported that security forces in Damascus had arrested a second would-be suicide bomber driving a car full of explosives near the site of the Mazraa bombing.
In the southern town of Daraa, where Syria's uprising began nearly two years ago, the Observatory said 18 people were killed in an airstrike on a field hospital, included eight rebel fighters, three medics, one woman and a young girl.
The conflict began in March 2011 with political protests against the government and has since evolved into a civil war between Assad's regime and hundreds of rebel groups seeking to topple it.
International diplomacy has failed to slow the fighting.
British Foreign Secretary William Hague said Thursday that his message to Assad is "it is time to go" and that the senseless killing must be brought to an end through a political process.