DAMASCUS/BEIRUT: A massive car bomb exploded near Syria’s ruling party headquarters in Damascus Thursday, killing at least 53 people and causing widespread destruction in one of the Syrian capital’s deadliest attack since the conflict erupted.

The blast occurred as opposition figures were meeting to try to hammer out guidelines for direct political negotiations with the Syrian government.

Syrian state media put the toll at 53 with more than 200 wounded. Anti-regime activists said 59 had died. In May, a double suicide bombing killed 55 people in the capital.

Three straight days of mortar attacks on the center of Damascus after recent rebel advances in the suburbs marked the most sustained rebel challenge in the heart of President Bashar Assad’s seat of power.

Within hours of the car bombing, two other bombs went off elsewhere in the city and a mortar attack struck the army’s central command. Thirteen people were killed by the other two bombs, activists said.

The day’s most devastating attack struck a main street on the edge of the central Mazraa neighborhood, near the headquarters of Assad’s ruling 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 burning car with hoses as lifeless and dismembered bodies lay in 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 had exploded at a security checkpoint between the Russian Embassy and the central Baath headquarters.

“It was huge. Everything in the shop turned upside down,” one local resident said. He said three of his employees had been wounded by flying glass that killed a young girl who had been 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.

Among those wounded by flying glass near the blast was Nayef Hawatmeh, the leader of the Democratic Front for the Liberation of Palestine, a Damascus-based Palestinian group.

An official at his office, about 450 meters from the site of the explosion, said Hawatmeh was wounded in the hands and face from flying glass and briefly taken to hospital.

There was no immediate claim of responsibility for the attack, although the most extreme of Syria’s rebel groups, the Nusra front, has claimed responsibility other suicide bombings that have struck regime targets but also killed civilians.

Such tactics have galvanized Assad’s supporters and made many other Syrians distrustful of the rebel movement,, most of whose fighting groups do not advocate such tactics.

“It is terrorism ... Is that what you call Islam?” one of the wounded told Syrian television.

“Is that the freedom you want? Is that the [rebel] Free Syrian Army?” asked another.

State TV called it a “terrorist” attack by a suicide bomber linked to the extremist Al-Qaeda network.

The attack was “carried out by armed terrorist groups linked to Al-Qaeda that receive financial and logistic help from abroad,” the Interior Ministry said in a statement, using government terminology for rebels.

The opposition also denounced the bombers as “terrorists.”

“Any acts targeting civilians with murder or human rights violations are criminal acts that must be condemned, regardless of the perpetrator or the justification,” the National Coalition said on its Facebook page.

It did, however, suggest that the regime allowed foreign terrorist groups to operate in Syria. “The terrorist Assad regime bears the most responsibility for all the crimes that happen in the homeland because it has opened the doors to those with different agendas to enter Syria and harm its stability,” the group’s statement said. The bombing was followed by a mortar bomb attack on a military headquarters, state television and the opposition activist group the Syrian Observatory for Human Rights reported, without giving any information on casualties.

The Observatory also said two other car bombs exploded next to security posts in the north Damascus district of Barzeh, killing five soldiers among eight people.

On Wednesday, two mortar bombs exploded next to a football stadium in Damascus, killing one player. A day earlier, two shells hit near one of Assad’s three palaces in the city, with some damage reported.

The attacks have thrown into doubt efforts by the Syrian National Coalition, meeting in Cairo Thursday, to reach an agreement on an offer by the opposition group’s chief to negotiate directly with the regime.

“The agenda is long and among the issues to be discussed is the initiative of Ahmed Moaz al-Khatib,” delegate Khaled Nasser told AFP.

Coalition chief Khatib has offered to talk to regime officials without “blood on their hands” – an initiative welcomed by the Arab League and the United States as well as Assad allies Iran and Russia.

But the Syrian National Council, a key part of the coalition, has rejected talks until Assad quits, and the regime says it will negotiate only without preconditions.

U.N.-Arab League peace envoy Lakhdar Brahimi, who Wednesday extended his mission by six months, has urged support for the plan, saying talks would “be a start to exiting the dark tunnel.”

In other violence Thursday, the Observatory said 18 people were killed in an airstrike on a field hospital in the southern town of Deraa, where Syria’s uprising began nearly two years ago.

The dead included eight rebel fighters, three medics, one woman and one young girl, according to the Observatory.