A car bomb targeting the headquarters of Syria's ruling party kills at least 53 people in central Damascus according to state televison.

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Syrian state media said over 200 people had been injured in what it described as a suicide bombing by "terrorists" battling President Assad.

Britain-based Syrian Observatory for Human Rights said most of those killed in the blast which happened near an office of President Bashar al-Assad's ruling Baath Party were civilians, though the death toll included security forces.

Syrian television broadcast footage of at least four bodies lying beside a main street and firefighters dousing the charred remains of dozens of burning vehicles. Black smoke billowed into the sky.

It also reported that mortar rounds exploded near army command in Damascus, but with no casualties.

Central Damascus has been relatively insulated from the nearly two-year conflict which has killed around 70,000 people across the country, according to the United Nations.

Attack on Assad stronghold

But rebels who control districts to the south and east of the capital have been attacking Assad's power base for nearly a month, and have struck with devastating bombings several times in the last year.

Al Qaeda-linked rebel group Jabhat al-Nusra has claimed responsibility for several of those attacks.

Thursday's blast, which activists said was followed by at least three other explosions elsewhere in Damascus, sent a pall of black smoke billowing into the sky above the Mazraa district.

Russia's Itar-Tass news agency quoted a diplomat as saying the blast blew out windows at the Russian embassy compound, which faces on to the road where the bomber struck, but no employees were wounded.

"The building has really been damaged... The windows are shattered," the diplomat said.

The observatory said the car bomb detonated near a building of the ruling Baath Party, about 200 metres south of the Russian Embassy compound.

Crater ripped in road

A correspondent for Syrian state television said he saw seven body bags with corpses in them at the scene. He said he counted 17 burnt-out cars and another 40 that were destroyed or badly damaged by the force of the blast, which ripped a crater 1.5 metres deep into the road.

The official SANA news agency said casualties included children at a nearby school in Mazraa, which it described as a busy residential district of the capital.

Activists reported at least two further blasts in the city after the Mazraa explosion. The observatory said two car bombs exploded outside security centres in the north-eastern district of Barzeh, but there were no details of casualties.

The blast comes a day bafter Syria's state-run news agency said two mortars have exploded inside a soccer stadium in central Damascus, killing one player and injuring several.

The SANA agency said the mortars landed in the Tishrin Stadium in the central Baramkeh district during soccer practice.

It said one player from the Homs-based al-Wathbah club was killed.

Assad 'cannot be part of peace plan'

The opposition Syrian National Coalition, meeting in Cairo on Thursday, said in a communique it is willing to negotiate a peace deal to end the war but President Assad cannot be a party to any settlement.

The communique omitted a direct demand for Assad's removal, in a softening of tone from past positions that insisted the president must go before there could be any talks.

The document, to be debated at a meeting of the opposition alliance's leadership starting on Thursday, said Assad and his cohorts must be held accountable for bloodshed and that any peace deal must be under the auspices of the United States and Russia.

Russia remains one of the Assad regime's last foreign allies and the Syrian Foreign Minister Walid al-Moualem is due for talks in Moscow.