Syrian state media say at least 53 people have been killed and more than 200 wounded in a car bombing near the headquarters of Syria's ruling Ba'ath Party in central Damascus.
A Britain-based activist group said most of the dead from Thursday's blast were civilians, and that some security force members were killed.
State media called the blast a "terrorist bombing" that struck a densely populated area near the al-Mazraa neighborhood. TV footage showed bloodied and charred bodies lying in the street.
Russian news agencies said the bombing blew out windows at the Russian embassy.
There was no immediate claim of responsibility for the attack, which came amid a series of bombings Thursday in the capital, including a mortar attack on a military facility.
Central Damascus has been relatively insulated from the nearly two years of violence in Syria.
Meanwhile, the Syrian opposition is meeting Thursday in Cairo to discuss an initiative to possibly hold peace talks with President Bashar al-Assad's government.
On Wednesday, Syrian activists said rebels shot down a warplane outside Damascus, shortly after an airstrike killed at least 20 people in the same area.
The opposition has offered to talk with regime officials who have "no blood on their hands." The Assad government has said it is ready to negotiate with the opposition but without pre-conditions.
Amateur video circulated by the Syrian Observatory for Human Rights showed a warplane firing rockets and then plummeting to the ground in Hamouriya after apparently being hit by rebel machine-gun fire. The shoot-down could not be independently verified.