TRIPOLI, Lebanon -- At least three car bombs roiled Damascus on Thursday, including a powerful blast near the downtown headquarters of President Bashar al-Assad's ruling party and the Russian Embassy that witnesses said shook the neighborhood like an earthquake. Antigovernment activists described the bombings as some of the worst to hit the city in the nearly two-year-old conflict and said at least 72 people had been killed, mostly civilians.
Witnesses, including people who had been living near the ruling party headquarters in the Mazraa district, said the bombings were eroding what little confidence they had left that Mr. Assad's forces could preserve at least some semblance of normalcy in Damascus, the Syrian capital, where armed insurgents have attacked with increasing brazenness.
There was no immediate claim of responsibility. The main umbrella opposition group seeking to depose Mr. Assad condemned the bombings as it convened a meeting in Cairo. It was unclear whether the blasts had been timed to the Cairo meeting.
Syria's state-run SANA news agency described the blasts as the work of armed terrorist groups, its standard terminology for the insurgency. SANA said the victims included children and students and hundreds of people had been wounded. It said the Foreign Ministry had sent letters to Ban Ki-moon, the United Nations secretary general, and the Security Council, urging that the body "adopt a firm stance which proves its commitment to combating terrorism regardless of its timing or place."
Some witnesses contacted in Damascus reported insurgent attacks and explosions elsewhere in the city on Thursday, including mortar rounds aimed at the Defense Ministry's headquarters, central Umayyad Square and a park in a heavily protected affluent neighborhood, Abu Roumana. Earlier this week insurgent fighters lobbed mortar rounds that damaged one of the presidential palaces and killed a soccer player practicing inside a stadium.
"It is the first time to feel we are living in a war condition," said a 30-year-old Mazraa resident named Anas, who lives with his family in a house behind the headquarters of Mr. Assad's Baath Party. "Today I saw what was happening in Baghdad in my city, Damascus. This is not the Damascus I know."
He said the Mazraa bombing "was similar to an earthquake -- my house's windows were broken."
The Syrian Observatory for Human Rights, an anti-Assad group based in Britain that has a network of contacts in Syria, reported that at least 59 people were killed by the Mazraa district bomb, which the group described as a booby-trapped car next to a military checkpoint. It said at least 16 of the dead were members of the security forces.
It said at least 13 other people in Damascus were killed -- 10 of them in the security forces -- in two other car bombings near checkpoints in the Barzeh district, in the northeast part of the capital.
Syrian rebels have been entrenched for months in suburbs south and east of Damascus, but they have been unable to push far into the center, although they have hit the area before with occasional mortar shells and increasingly frequent car bombs.
Such indiscriminate attacks have risked killing passers-by, exposing the rebels to accusations that they are careless with civilian life and property. Many Damascus residents have remained undecided in the civil war and fear that their ancient city will be ravaged like Aleppo and other urban centers to the north. At the same time, the government has decimated pro-rebel suburbs with airstrikes and artillery, leaving vast areas depopulated and traumatized.
Some outside experts speculated that the bombings on Thursday had been carried out by the more militant extremists among the rebels in order to weaken the government's argument that it offers security, at least to the middle- and upper-income neighborhoods of Damascus.
"The opposition will have to go in and mess that up if they want people to leave the regime," said Joshua M. Landis, director of the Center for Middle East Studies at the University of Oklahoma and author of the Syria Comment blog, which has tracked the conflict. "Messing up Damascus and forcing the city people to take sides is the name of the game."
The National Coalition of Syrian Revolutionary and Opposition Forces, the main Syrian umbrella group for the opposition, denounced the car bombings and other mayhem that killed civilians in Damascus on Thursday, saying in a statement that it "holds the Assad regime responsible for them."
The group issued the statement as it was meeting in Cairoto talk about a negotiated settlement to the conflict, which has claimed an estimated 70,000 lives since it began as a nonviolent uprising against Mr. Assad in March 2011.
The meeting focused mainly on recent proposals by the coalition's leader, Sheik Ahmad Moaz al-Khatib, to talk with representatives of the Assad government. Those proposals have been criticized by some of Mr. Khatib's colleagues, who contend that they have only emboldened Mr. Assad.
Participants at the Cairo meeting made clear that at least so far, Mr. Khatib was speaking for himself.
"Per his own words it is not a formal initiative," said Yasser Tabbara, a legal adviser to the coalition. "It is an idea he had, and now he is seeking some sort of a sanction for it, through the general assembly of the coalition." Mr. Tabbara added, "It is no secret to anyone that the fact that Sheik Khatib took this unilateral initiative or unilateral approach was a problem."
Some members of the coalition said the dispute remained largely theoretical. No one believes that officials of Mr. Assad's government will talk with the armed opposition about any political solution other than the opposition's complete surrender, several said.
Monzer Makhous, the coalition's representative in Paris, who was attending the Cairo meeting, said he believed that the intended audience for Mr. Khatib's overtures for talks was not the Assad government but its Russian backers. The Russians have blocked United Nations actions against Mr. Assad and consistently accused the opposition of a refusal to negotiate, and Mr. Makhous said American and other Western officials had encouraged Mr. Khatib to appear open to talks primarily as a tactic to try to soften the Russian resistance to international action.
French and American diplomats told the Syrian coalition that this was "very important," Mr. Makhous said. "It is to try to make some movement in the international scene."
Anne Barnard reported from Tripoli, and Rick Gladstone from New York. Reporting was contributed by Hwaida Saad and Hania Mourtada from Tripoli, David D. Kirkpatrick and Mayy el Sheikh from Cairo, Alan Cowell from London, Christine Hauser from New York, and an employee of The New York Times from Damascus, Syria.
Correction: February 21, 2013, Thursday
This article has been revised to reflect the following correction: Because of an editing error, an earlier version of this article misidentified the country that the bylined reporters were filing from. It is Lebanon, not Libya.
This article originally appeared in The New York Times.