Two huge explosions tore through the Syrian capital Damascus, leaving dozens dead and up to 100 wounded in an attack which officials and state television blamed on al-Qaida suicide bombers.

Opposition activists however pointed the finger at the regime of Bashar al-Assad, accusing it of plotting the blasts to reinforce its narrative that, rather than protesters seeking democratic reform, the nine-month uprising is being driven by Islamists bent on chaos.

The car bombs were detonated shortly after the arrival of Arab League observers in Damascus to monitor the regime's promise to end its bloody crackdown on the protest movement. It is the first time large bombs have been detonated in the capital since the revolt began in March.

Officials said around 40 people, mostly civilians, were killed. "We said it from the beginning, this is terrorism. They are killing the army and civilians," said the deputy foreign minister, Faysal Mekdad.

But opponents of the regime were quick to question this. Omar Idilbi, a member of the opposition group the Syrian National Council, described the explosions as "very mysterious" because they happened in heavily guarded areas difficult to reach by car.

"The presence of the Arab League advance team of observers pushed the regime to give this story in order to scare the committee from moving around Syria," he told the Associated Press. It was also "an attempt to make the Arab League and international public opinion believe that Syria is being subjected to acts of terrorism by members of al-Qaida," he said.

The blasts appear to have caused significant damage to two security buildings, the state security directorate and a branch of military intelligence, in the heart of Damascus, where an advance party of 13 Arab League observers arrived on Thursday. State TV showed scenes of mutilated bodies, burnt-out cars and damaged buildings in the upmarket district of Kfar Sousa. Syrian officials have insisted for many months that al-Qaida inspired groups are conducting sectarian attacks on members of the country's ruling Allawite sect and other minorities. State media commentators repeated that line, while broadening the list of potential culprits for the bombing to include Israel and the US.

Ahead of the Arab League delegation's arrival, Syrian foreign minister Walid Muallem predicted that the regime's claims would be vindicated. "There are many countries in the world who don't wish to admit the presence of terrorist armed groups in Syria," he said at a press conference. "They will come and see that they are present."

State media had warned earlier this week that Syria had received information from Lebanon about an imminent attack. Soon after the explosions the delegation was taken to the scene, in one of the most tightly controlled areas of the capital.

One Damascus resident, who wished to remain anonymous, said a road near the state security building had been closed 10 minutes before the first bomb detonated and that the security presence in the area had been much higher than normal.

Former Lebanese prime minister Saad Hariri, whose father Rafiq Hariri was killed by a Beirut car bomb six years ago, accused Syrian officials of orchestrating the attack. "I believe that the blast was engineered by the Syrian regime, just like the Syrian National Council declared," he wrote on Twitter. Hariri, who remains in exile travelling between Riyadh, Paris and Canada, said any warning passed to Damascus from Beirut was "fabricated by the Syrian ministry and some of its tools in Lebanon".

Washington later urged that the blasts should not to hinder the work of the Arab League's monitoring mission, which is aiming to broker discussions between the Assad regime and opposition groups.

The US has already warned of further, unspecified, measures against Syria if the Arab League is not given access to key areas of the country and the signed agreement is not implemented.

The mission was finally allowed into Syria this week after being delayed for close to a month since the Arab League announced sanctions against Damascus, infuriating regime officials.

At least 5,500 civilians, some deserters from the military, have been killed during the uprising. An estimated 2,000 security force members have also died.

The revolt began in the southern city of Deraa in March, inspired by the popular movements elsewhere in the Arab world that had by then overthrown Tunisian leader Zine El Abidine Ben Ali and his Egyptian counterpart, Hosni Mubarak, and imperilled Libya's Muammar Gaddafi.

It continued as a campaign of attrition between demonstrators and security forces until the summer, when an armed rebellion began in the western city of Homs.

Since then, Homs has descended into sectarian fighting, with Sunni defectors from the military and Sunni civilians clashing with members of the Allawite sect, which is protected by loyalist forces.

Northern Syria is also a scene of fierce clashes, though with less of a sectarian dimension. Defectors and security forces fought pitched battles there this week in the Jabal al-Zawiya area near the city of Idlib, which has been a hub for the nascent guerilla force, the Free Syria Army. Damascus has remained largely under the control of the regime, with the exception of the satellite suburb of Douma, where the Free Syria Army is active.