Photo: Contributed - CBC

This photo released by the Syrian official news agency SANA, shows Syrian security agents next to a vehicle on fire following a huge explosion that shook central Damascus, Syria. (SANA/Associated Press)

A car bombing near Syria's ruling party headquarters in Damascus killed 53 people on Thursday, according to state media, while mortar rounds exploded near the army's central command in the city.

It was the third straight day of attacks on the centre of the capital, among the deepest and fiercest on the heart of Bashar Assad's seat of power during the civil war.

The car bombing was the deadliest attack inside Damascus in nine months and within hours, two other bombings and a mortar attack on the military compound followed.

While no one group has claimed responsibility, the attacks suggest that rebel fighters who have gotten bogged down in their attempts to storm the capital are resorting to guerrilla tactics to loosen Assad's grip on the capital.

The day's deadliest attack struck a main street on the edge of central Mazraa neighbourhood, near the headquarters of Assad's Baath party and the Russian Embassy, as well as a mosque, a hospital and a school.

TV footage of the blast site showed firemen dousing a flaming car with hoses and lifeless and dismembered bodies blown into the grass of a nearby park. The state news service, SANA, published photos showing a large crater in the middle of the rubble-strewn street and charred cars holding blackened bodies.

Witnesses at the scene said a car exploded at a security checkpoint between the Russian Embassy and the central headquarters of Assad's ruling party.

"It was huge. Everything in the shop turned upside down," one local resident said. He said three of his employees were injured by flying glass that killed a young girl who was walking by when the blast hit.

"I pulled her inside the shop but she was almost gone. We couldn't save her. She was hit in the stomach and head," he said, speaking on condition of anonymity for fear of retribution for speaking with foreign media.

Ambulances rushed to the scene of the blast, which shattered windows and sent up a huge cloud of smoke visible throughout much of the city, witnesses said.

State TV called it a "terrorist" attack by a suicide bomber. It said at least 53 people were killed and more than 200 wounded.

The Britain-based activist group the Syrian Observatory for Human Rights said at least 42 people were killed, most of them civilians. Some members of the Syrian security forces were also killed, it said.

There was no way to immediately reconcile the differing death tolls.

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Hubbard reported from Beirut. Associated Press writer Zeina Karam contributed reporting from Beirut.