A huge car bomb exploded on Thursday near ruling party offices in Damascus, killing nearly 60 people and causing widespread destruction in the Syrian capital's deadliest attack since the civil war erupted.
The bombing, blamed on "terrorists" by both the regime and its opponents, rocked the city centre and sent thick smoke scudding across the skyline, shortly before a mortar attack on a nearby military headquarters.
The attacks came as the opposition umbrella group, the National Coalition, met in Cairo to discuss proposals to hold conditional talks with President Bashar al-Assad's regime.
Images of charred bodies lying next to mangled vehicles were aired on state television, which said children were among the 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 it 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 and wounded dozens.
There was no immediate claim of responsibility.
"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.
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 bombing was followed by a mortar attack on a military headquarters, state television and the Observatory 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.
The attacks come 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.
"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 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.
"It is important that that offer is responded to with serious negotiations by the Assad regime," Hague told reporters in Beirut, but also called on Assad to quit.
"It's time to go," he said, when asked what his message was for Assad.
UN-Arab League peace envoy Lakhdar Brahimi, who on Wednesday extended his mission by six months, has urged support for the plan, saying talks would "be a start to exiting the dark tunnel."
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.
FSA chief of staff General Selim Idriss told AFP the rebels were giving Hezbollah a 48-hour deadline to stop the attacks and "as soon as the ultimatum ends, we will start responding to the sources of fire."