A huge car bomb near the headquarters of Syria's ruling party killed nearly 60 people and caused widespread destruction in the deadliest attack to hit Damascus since the civil war erupted, AFP reports.
Thursday's bombing, which both the regime and its opponents blamed on "terrorists", rocked the city centre and sent thick smoke scudding across the skyline, shortly before a mortar bomb attack on a nearby military headquarters.
The attacks came as opposition umbrella group the National Coalition met in Cairo to discuss proposals for conditional talks with President Bashar al-Assad's regime.
State television showed images of charred bodies next to mangled vehicles and said children were among those wounded in the blast, near a school in the central district of Mazraa.
The attack was "carried out by armed terrorist groups linked to Al-Qaeda that receive financial and logistic help from abroad", the foreign ministry said, using government terminology for rebels.
Police said the bomb exploded at 16 November Square near the Baath party's head offices, and Russian news agencies reported the windows of Moscow's embassy were blown out, although no staff were hurt.
The Syrian Observatory for Human Rights said the blast killed at least 59 people, including 15 soldiers, and wounded more than 200.
That would make it the deadliest such attack in Damascus since bombers first targeted the city about a year ago. On May 10, 2012, 55 were killed in twin suicide bombings in the capital.
State media said the bombing killed 53 people.
There was no immediate claim of responsibility.
Among the lightly wounded was Democratic Front for the Liberation of Palestine chief Nayef Hawatmeh, the DFLP said. He was hit in the face and hands by flying glass in the blast 500 metres (yards) from his office.
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 Facebook.
The office of UN Secretary General Bank Ki-Moon said he condemned the bombings. "The secretary general renews his call on all parties to end the violence and respect international humanitarian law," said a spokesman.
The blast was followed by a mortar bomb attack on a military headquarters, state television and the Observatory reported, without giving information on casualties.
The Observatory also said two other car bombs exploded next to security posts in the north Damascus district of Barzeh, with five soldiers among eight people killed.
The attacks came two days after two mortar rounds exploded near Tishreen presidential palace in Damascus, increasingly targeted in the past year by suicide bombings, some claimed by the jihadist Al-Nusra Front.
They added urgency to the National Coalition meeting in Cairo, where discussions focused on an offer by the opposition group's chief to talk directly with the regime.
Coalition chief Ahmed Moaz al-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 any talks until Assad quits, and the regime says it will negotiate only without preconditions.
British Foreign Secretary William Hague urged Assad's regime to respond positively to Khatib's offer, while also calling on Assad to quit.
UN-Arab League peace envoy Lakhdar Brahimi, who on Wednesday extended his mission by six months, has also urged support for the plan.
At the United Nations, Russia accused US diplomats of blocking a Security Council condemnation of the attack. Other diplomats said however that Russia had refused to include language criticising Assad.
The UN says at least 70,000 people have been killed in the conflict, and the violence frequently spills across Syria's borders, spiking already simmering regional tensions.
On Wednesday, the rebel Free Syrian Army threatened to hit back at Hezbollah, the powerful Lebanese militant group that backs Assad, unless it stops shelling FSA-held territory from across the border.