Twin suicide car-bomb attacks have killed at least 55 people and wounded 372 in Damascus, Syrian officials say.
The blasts happened near a military intelligence building during morning rush hour. State TV showed burnt cars and two deep craters in the road.
The government and the opposition blamed each other for the attack - the deadliest in Damascus since the start of Syria's uprising 14 months ago.
The two sides are supposed to observe a ceasefire monitored by a UN team.
However, violence has continued unabated across the country, with the restive city of Homs shelled again overnight.
The UN Security Council condemned the blasts "in the strongest terms" and urged all parties to cease violence and abide by the terms of the six-point peace plan.
International peace envoy Kofi Annan - who brokered the plan - said the Damascus blasts were "abhorrent" and counter-productive.
Central Damascus is under the firm control of government forces, but the city has been hit by several bombings in recent months, often targeting security buildings or military convoys.
Thursday's attacks occurred in the southern suburb of al-Qazzaz shortly before 08:00 (05:00 GMT), as people were going to work.
The interior ministry said "foreign-backed terrorists" carried out the attacks, using two cars "loaded with more than 1,000kg of explosives and driven by suicide bombers".
"The house shook like it was an earthquake," Maha Hijazi, standing outside her home nearby, told the Associated Press.
The explosions damaged the facade of a 10-storey military intelligence building involved in the crackdown on the uprising against President Bashar al-Assad's rule.
It is part of a broader military compound of the Palestine Branch, one of the most feared among the more than 20 secret police organisations in the country, correspondents say.
The unit was originally set up in the 1950s to interrogate suspected Israeli spies. But over the past decade, it has evolved into the country's counter-terrorism unit, and is infamous for interrogations and torture, they say.
Large crowds gathered despite the security cordon, shouting slogans and chants in support of President Assad.
All the while, the emergency teams looked for body parts and bulldozers were at work clearing away some of the debris. The whole area looks like a wasteland, says the BBC's Lyse Doucet at the scene.
One Damascus resident, who gave his name as Ahmad, told the BBC the blasts were the biggest explosions he had ever heard.
"The whole of Damascus heard them. At first, I thought they were air strikes," he said.
The Norwegian head of the UN observer mission in Syria, Maj Gen Robert Mood, visited the site. He said the Syrian people did not deserve this "terrible violence".
In its statement, the UN Security Council condemned "in the strongest terms the terrorist attacks that occurred in Damascus, Syria, on 10 May, causing numerous deaths and injuries".
It added that "any acts of terrorism are criminal and unjustifiable, regardless of their motivation".
The US state department also condemned the attack, saying indiscriminate killing could not be justified. It called on the Syrian government to "fully and immediately implement the Annan plan".
The call was echoed by UK Foreign Secretary William Hague, who said the people of Syria were "suffering as a result of the repression and violence, which must come to an end".
The opposition says the attacks were carried out by the government itself to discredit rebel forces.
Meanwhile, the Syrian army bombarded the city of Homs overnight.
Opposition activists described the bombardment as some of the heaviest shelling in Homs for weeks.
Eleven UN monitors are now stationed there to try to implement a ceasefire.
The observers are in the country as part of the joint UN-Arab League peace plan and began deploying last month.
There are now 105 monitors in the whole country, but their presence has had no effect in quelling the violence. A total of 300 are due to be sent in the coming weeks.
The UN says at least 9,000 people have died since pro-democracy protests began in March 2011.