Suicide car bombers struck Damacus on Friday, officials said, killing 40 people, gutting buildings and sending human limbs flying in the bloodiest violence to hit Syria's capital in a nine-month-old uprising against President Bashar al-Assad.
The attacks, which state media blamed on al Qaeda, targeted two Syrian security sites and came a day after the arrival of Arab League officials to prepare for a monitoring team that will check whether Assad is implementing a plan to end the bloodshed.
Assad has deployed tanks and troops to try to crush a wave of protest against his rule inspired by other Arab revolts this year. Mainly peaceful rallies are now increasingly eclipsed by an armed insurgency against his military and security apparatus.
But Friday's blasts in central Damascus signalled a dramatic new escalation in violence, which Syrian authorities blame on armed groups they say have killed 2,000 soldiers and security force members since popular unrest broke out in March.
State television said over 150 people were wounded by the explosions. It broadcast footage of mangled bodies being carried in blankets and stretchers into ambulances with sirens wailing.
Television footage also showed bloodied streets littered with human remains, blackened car wrecks and other debris, as well as a row of corpses wrapped in sheets laid along a street.
State television said the attackers zeroed in on a state security administration building and a local security branch.
Syria has generally barred foreign media from the country, making it hard to verify accounts of events from either side.
Foreign Ministry spokesman Jihad Makdesi said the attacks were carried out by terrorists (trying) to sabotage the will for change in Syria, and followed warnings from Lebanon that al Qaeda fighters had infiltrated Syria from Lebanese territory.
There was no immediate claim of responsibility.
Assad's opponents said the attack could have been staged to drive home the government's argument. We have all sorts of suspicions that this could be organised by the regime itself, said Basma Qadmani, spokeswoman for the Syrian National Council.
The United Nations says Assad's forces have killed more than 5,000 people in their crackdown on the protests, which erupted in March instigated by uprisings that toppled autocratic leaders in Tunisia, Egypt and Libya over the course of the year.
The intensifying violence on both sides in Syria has raised fears that the country is drifting towards civil war. Downtown Damascus and Syria's second city Aleppo hitherto had largely escaped the turmoil now common in many other cities and towns.
First I heard an explosion and then, all of a sudden, I saw human limbs flying everywhere, said one man interviewed by Syrian television near the site of the attack in Kafr Sousa.
His head and face were covered in bandages.
State media said the Arab League delegation, which will be seeking assurances of free movement for 150 monitors due to arrive in Syria by the end of the month, had visited the sites of the explosions to inspect the damage.
The monitors are supposed to verify Syria's implementation of an Arab League peace plan it agreed six weeks ago, which stipulates a withdrawal of tanks and troops from the streets of protest-hit cities and towns and their surroundings, release of prisoners and reform-minded dialogue with the opposition.
Arab League sources have said the advance team, led by top League official Samir Seif al-Yazal, comprises a dozen people, including financial, administrative and legal experts.
Activists say Assad, 46, is still trying to suppress public dissent with military force despite being hit with European Union and Arab League sanctions, and his avowed commitment to the peace plan.
The Syrian Observatory for Human Rights said Assad's forces carried out major assaults in the northern and southern provinces this week, apparently trying to quell serious opposition to strengthen his hand before the monitors settle in.
Syrian troops surrounded and killed 111 people on Tuesday in the northern province of Idlib, in the deadliest single assault since the uprising erupted, according to the Observatory.
France called Tuesday's killings in Idlib an unprecedented massacre. The United States said Syrian authorities had flagrantly violated their commitment to end violence while Assad's former ally Turkey condemned Syria's policy of oppression which has turned the country into a bloodbath.
Damascus says more than 1,000 prisoners have been freed since the Arab plan was agreed and the army has pulled out of cities. Anti-Assad activists deny any such pullout has occurred.
The government has promised a parliamentary election early next year as well as constitutional reform that might loosen the Baath Party's 48-year grip on power.
Syrian pro-democracy activists are deeply sceptical about Assad's commitment to the plan. If implemented, it could embolden demonstrators demanding an end to his 11-year rule, which followed three decades of domination by his father.
The British-based Avaaz rights group said on Thursday it had evidence of more than 6,237 deaths of civilians and security forces in the conflict, 617 of them under torture. At least 400 of the dead were children, it added.
(Writing by Dominic Evans; Editing by Mark Heinrich)