Syria says twin suicide bombings in Damascus kill 44
At least 44 people have been killed and more than 150 injured in two suicide car bombings in the heart of Syria's capital, Damascus, officials say.
State TV said suspected al-Qaeda militants had targeted two security service bases in the Kafr Sousa area.
But the opposition said the attacks - which came a day after Arab League observers arrived - were staged by the government to justify its crackdown.
Correspondents say the attacks mark a significant escalation of violence.
The Arab League team are tasked with monitoring whether the government complies with a peace agreement that orders all troops to withdraw from the streets, with the aim of ending the violence.
But human rights and opposition activists said the killings continued on Friday, with security forces shooting dead at least 12 civilians.
More than 5,000 people have been killed and thousands more detained since anti-government protests erupted in March, the UN says.
The BBC's Jim Muir in neighbouring Lebanon says we may never know who carried out the bombings - but that they were absolutely against the interests of the opposition, and played strongly into the hands of the government.
The two explosions happened within minutes of each other on Friday morning.
Within minutes, state TV said two attacks had been carried out by suicide bombers driving vehicles packed with explosives against the General Security Directorate and another branch of the security services in the upmarket Kfar Sousa district, south-west of the city centre.
"Preliminary investigations showed al-Qaeda was responsible," it added.
Video footage was broadcast of heavily damaged buildings, with rescue workers combing through burnt buildings and blood-stained debris, and ambulances taking the injured away.
The state-owned news channel, al-Ikhbariya al-Suriya, said the first car bomb exploded outside the offices of an unspecified security agency.
When guards at a nearby compound housing the General Security Directorate went to inspect the aftermath of the first blast, the driver of another vehicle rammed the main gates and detonated the bomb it was carrying, the channel said.
"The explosions shook the house, it was frightful," Nidal Hamidi, a Syrian journalist who lives in Kfar Sousa, told the Associated Press news agency.
Gunfire was heard immediately following the explosion and windows up to 200m (670ft) away were shattered, Mr Hamidi said.
Deputy Foreign Minister Faisal Mekdad told reporters: "On the first day after the arrival of the Arab observers, this is the gift we get from the terrorists and al-Qaeda.
"But we are going to do all we can to facilitate the Arab League mission."
Mr Mekdad was accompanied by the Arab League's Assistant Secretary General, Samir Seif al-Yazal, who said the nine-strong advance team of monitors would not be deterred.
"We are here to see the facts on the ground," he added. "What we are seeing today is regretful, the important thing is for things to calm down."
The US state department condemned the attacks in Damascus but said they must not deter the Arab League observers from doing their work.
Syrian foreign ministry spokesman Jihad al-Makdissi told the BBC that Lebanon had warned it two days ago that "al-Qaeda groups had infiltrated inside Syria through the town of Arsan".
But the Syrian Revolution General Commission - a coalition of 40 opposition grassroots groups - accused the government of fabricating the attacks and urged the Arab League observers to investigate.
With a solid security presence, Damascus has largely escaped the violence and protests that have flared in central and northern provinces, although there have been protests in suburbs.
Our correspondent says the Arab observers gained a first impression entirely untypical of the long story so far.
They were given a graphic reinforcement of the regime's contention, that it is all an issue of armed terrorism, not a popular uprising.
Whoever was behind it, he says, the result will be to polarise opinion, aggravate sectarian sentiment, and envenom the climate even more dangerously that it already is.
Omar Idilbi, a member of the main opposition coalition, the Syrian National Council, said the explosions were "very mysterious because they happened in heavily guarded areas that are difficult to penetrate by a car".
Human rights groups and opposition activists said there were more killings by security forces on Friday.
The London-based Syrian Observatory for Human Rights said six people had died in the central city of Homs, three in the southern province of Deraa, and three in the eastern oil city of Deir al-Zour.
It also published a video showing the bodies of at least 49 men, whom it said had been killed in the Jabal al-Zawiya area of Idlib province, where troops have launched assaults on army defectors and several villages.
The Local Co-ordination Committees of Syria put the number of dead on Friday at 16.