Two suicide car bombers killed more than 55 people and wounded 372 in the Syrian capital Damascus, the Syrian Interior Ministry said in a statement on government-owned TV on Thursday.
The rush hour explosions hit a district that houses a well-known military intelligence complex involved in Syrian President Bashar Assad's crackdown on a 14-month-old uprising. State media said earlier the majority of the casualties were civilians.
In addition to the violence in Syrian capital, opposition activists claimed that troops loyal to Assad gunned down at least 19 people across the country on Thursday, according to Al Jazeera, raising the death toll that the UN said already stands at over 9,000 people.
International mediator Kofi Annan condemned the deadly twin bomb explosions and called on Syrian forces and opposition fighters to halt the bloodshed in line with an agreed month-old ceasefire.
Annan said in a statement issued in Geneva: "These abhorrent acts are unacceptable and the violence in Syria must stop."
"Any action that serves to escalate tensions and raise the level of violence can only be counter-productive to the interests of all parties," he said.
Syrian television blamed "terrorists" for the morning rush-hour blasts, which were the deadliest to hit the capital since the revolt began. It showed mangled, burnt and smouldering vehicles, some containing incinerated human remains, and said more than 170 people were wounded.
One of the explosions wreaked damage over a wide area, punching a crater 3 meters deep into the tarmac. Bloodied corpses and body parts could be seen on the road.
The explosions occurred a day after a bomb blast near UN observers monitoring a UN ceasefire deal - which state forces and rebels have both violated - and two weeks after authorities said a suicide bomber killed at least nine people in Damascus.
"This is yet another example of the suffering brought upon the people of Syria from acts of violence," said Major-General Robert Mood, the head of the UN monitors who toured the site.
"We have seen it here in Damascus and we have seen it in other cities and villages across the country... I call on everyone within and outside Syria to help stop this violence."
Opposition to Assad, which began with peaceful protests in March last year, has grown increasingly militarized and UN Secretary General Ban Ki-moon said this week he was worried by an "alarming upsurge" in bombings.
Damascus residents said Thursday's explosions struck in the same area almost simultaneously, shortly before 8 a.m. (0500 GMT). Video footage sent to media by activists showed two columns of smoke, one of them forming a dark heavy cloud.
State television showed the crater in the city's southern ring road and at least one overturned lorry. Walls of buildings on either side of the wide avenue had collapsed.
Pro-Assad Syrians express anger at Gulf States
Shooting could be heard in the background of the footage, filmed shortly after the blasts.
A man walking around the wreckage pointed at the charred remains of cars. "Is this freedom?" he said. "This is the work of the Saudis," he added, referring to the Gulf state that has advocated arming rebels seeking to oust Assad.
Nadine Haddad, a candidate in Monday's parliamentary election which was boycotted by most opposition figures, blamed Qatar's Prime Minister Sheikh Hamad bin Jassim al-Thani, who also says Syrian rebels should get weapons.
"I am addressing Sheikh Hamad and I tell him shame on you. You are now destroying the Syrian people, not the Syrian regime. You are killing children going to school," she said.
The British-based Syrian Observatory for Human Rights said at least one of the explosions was caused by a car bomb and that the target was intelligence buildings.
The blasts caused limited damage to the facade of the nearby Palestine Branch Military Intelligence complex, one resident told Reuters. The Palestine Branch is one of the most feared of more than 20 secret police organizations in the country.
Jpost.com staff contributed to this report