TRIPOLI, Lebanon — At least three car bombs roiled Damascus on Thursday, including a powerful blast near the downtown headquarters of President Bashar 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 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 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 UN 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.’’
Asked about the bombings, Martin Nesirky, Ban’s spokesman at the United Nations, described them as appalling attacks that underscore ‘‘the need to end the violence and move onto a political track.’’ The United Nations and Arab League also announced that Lakhdar Brahimi, their special Syria peace envoy whose mandate was due to expire on Friday, will remain in that position at least through the end of 2013.
Some witnesses contacted in Damascus reported insurgent attacks and explosions elsewhere in the city 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. 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 Assad’s Ba’ath Party. ‘‘Today I saw what was happening in Baghdad in my city, Damascus. This is not the Damascus I know.’’
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.
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 Cairo to talk about a negotiated settlement to the conflict, which has claimed an estimated 70,000 lives since it began as a nonviolent uprising against 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 Khatib’s colleagues, who contend that they have only emboldened Assad.
Participants at the Cairo meeting made clear that at least so far, 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.’’
Some members of the coalition said the dispute remained largely theoretical. No one believes that officials of 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 Khatib’s overtures for talks was not the Assad government but its Russian backers. The Russians have blocked UN actions against Assad and consistently accused the opposition of a refusal to negotiate, and Makhous said US and other Western officials had encouraged Khatib to appear open to talks primarily as a tactic to try to soften the Russian resistance to international action.
French and US diplomats told the Syrian coalition that this was ‘‘very important,’’ Makhous said.