Suicide bombers hit two security service bases in Damascus on Friday, killing 44 people, in attacks the regime blamed on Al-Qaeda but which the opposition said were the work of the regime itself.
The attacks were the first against the powerful security services in the heart of the capital since an uprising against President Bashar al-Assad began in March.
"Forty-four people, civilians and security forces, were killed and 166 others injured in the two terrorist operations," the interior ministry said, as an advance team made preparations for Arab League observers to oversee a plan to end the bloodshed.
The material damage was considerable, the statement on state television said, charging that "the hands of Al-Qaeda were behind" the attacks.
One bomber tried to ram a vehicle packed with explosives into the compound of the General Security Directorate, Syria's most important plain-clothes security service, in the Kfar Suseh neighbourhood of Damascus, witnesses said.
A second blew up a vehicle outside a nearby military intelligence building.
Television showed pictures of a huge crater at one of the bomb sites and pools of blood on surrounding pavements.
Bystanders were seen carrying away charred and mangled bodies wrapped in makeshift shrouds.
"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," Deputy Foreign Minister Faisal Meqdad told reporters at one of the bomb sites.
Asked to comment on suggestions that the bombings had been engineered by the regime itself, Meqdad shot back: "Anyone who makes such allegations is a criminal."
The opposition group, the Syrian National Council, made just such a claim.
"The Syrian regime, alone, bears all the direct responsibility for the two terrorist explosions," said an SNC statement received in Nicosia.
"It wanted to send a warning message to the observers for them not to approach security centres."
The regime is trying to give the world the impression "that it faces danger coming from abroad and not a popular revolution demanding freedom and dignity," the statement added.
The SNC also accused the regime of having transferred "thousands of prisoners (from jails) to fortified military barracks," to which the observers would not have access.
While not specifically rejecting Syria's account of what happened, France accused the regime of trying to hide its brutal tactics from foreign observers.
"We still don't have any details on the origin of these attacks," foreign ministry spokesman Bernard Valero said when asked whether Paris shared suspicions the regime might have staged the bombings.
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However, Valero said France was more generally concerned that Syria had been acting for several days "to mask the reality of the repression, notably by transferring political prisoners to secret jails.
"Bashar al-Assad is wrong if he thinks he can once again trick the international community by play acting, lies and procrastination," he said.
State Department spokesman Mark Toner said the "United States condemns in the strongest terms the bombings today in Damascus."
He also said it was crucial the attack "not impede the critical work of the Arab League monitoring mission."
Traditional Assad ally Russia condemned the "barbaric terrorist act."
"We must note that the extremist forces are rejecting an inter-Syrian settlement, through national dialogue", and instead are trying to achieve their objectives by "instigating confrontation and armed provocations", the foreign ministry said.
Ban Ki-moon's spokesman said the UN leader was "gravely concerned" about the escalating and urged the government to "fully and speedily" implement the Arab League peace plan, while Britain's foreign ministry condemned the attacks and "the loss of life and injury."
Syria's deputy foreign minister was accompanied to the site by Arab League Assistant Secretary General Samir Seif al-Yazal, head of the advance team, which only arrived on Thursday.
"What has happened is regrettable but the important thing is that everyone stay calm," Yazal told reporters.
"We are going to press on with our work. We have started today, and tomorrow (Saturday) we will meet (Foreign Minister) Walid Muallem."
Yazal's nine-strong team is making logistical arrangements for the arrival of the first observers, who will eventually number between 150 and 200.
In Cairo, the Arab League's Ahmed ben Helli said the mission will head to Syria on Monday, grouping more than 50 experts in politics, human rights, military issues and crisis management, the official MENA news agency reported.
The mission is part of an Arab plan endorsed by Syria on November 2 that also calls for the withdrawal of the military from towns and residential districts, a halt to violence against civilians and the release of detainees.
Muallem has said he expects the Arab observers to vindicate his government's contention that the unrest is the work of "armed terrorists," not overwhelmingly peaceful protesters as maintained by Western governments and human rights watchdogs.
Syria says more than 2,000 security force personnel had been killed in attacks by armed rebels since March.
But opposition leaders have charged that Syria's agreement to the mission after weeks of prevarication was a mere "ploy" to head off a threat by the Arab League to go to the UN Security Council over a crackdown, which the world body says has left more than 5,000 people dead since March.
There was no let-up in the bloodshed on Friday with human rights activists reporting at least 14 civilians killed by security force fire.
Eight were killed in Homs, two in Hama, two in the Damascus suburb of Douma, one in Daraa and one in Idlib.