Two suicide car bombers killed 55 people and wounded 372 in Damascus on Thursday, state media said, the deadliest attacks in the Syrian capital since an uprising against President Bashar al-Assad began 14 months ago.
The blasts further shredded a ceasefire that was declared by international mediator Kofi Annan on April 12 but which has failed to halt bloodshed pitting Assad's security forces against peaceful demonstrators and an array of armed insurgents.
Opposition leaders said Annan's peace plan was dead, while Western powers insisted it remained the best way forward.
Annan himself condemned the "abhorrent" bombings and urged all parties to halt violence and protect civilians. "The Syrian people have already suffered too much," he said in a statement.
Syria's foreign ministry said the attacks were a sign that the major Arab state was facing foreign-backed terrorism and urged the U.N. Security Council to combat countries or groups supporting such violence.
"Syria stresses the importance of the UNSC taking measures against countries, groups and news agencies that are practicing and encouraging terrorism," the state news agency SANA quoted the ministry as saying in a letter to the U.N. body.
The near-simultaneous explosions hit the al-Qazaz district just before 8 a.m. (0500 GMT), residents said. One punched a crater three meters (10 feet) deep in the city's southern ring road. Bloodied corpses and body parts could be seen on the road.
State television also showed at least one overturned lorry. Walls of buildings on each side of the avenue had collapsed.
One resident reported limited damage to the facade of the nearby Palestine Branch Military Intelligence centre, one of the most feared of more than 20 Syrian secret police agencies. The huge walled complex was targeted by a 2008 bombing that killed 17 people and which authorities blamed on Islamist militants.
The opposition Syrian Observatory for Human Rights put the death toll from the bombings at 59 and said most of them worked for the security forces. No group has claimed responsibility.
The Interior Ministry vowed to "chase down the criminal killers and those who help or house them in their dens". It also appealed to citizens to pass on any information that might help.
MOUNTING DEATH TOLL
Rami Abdulrahman, head of the British-based Observatory, said 849 people - 628 civilians and 221 soldiers, of whom 31 were defectors - had been killed since the April 12 truce accord. The toll did not include Thursday's deaths.
The attacks occurred a day after a bomb exploded near U.N. observers monitoring the ceasefire, 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 (Thursday's attacks) is yet another example of the suffering brought upon the people of Syria from acts of violence," said Major-General Robert Mood, leader of the U.N. monitors, who visited the scene.
Opposition to Assad, which began with peaceful protests in March 2011, has grown increasingly militarized. U.N. Secretary-General Ban Ki-moon said on Wednesday there was only a narrow window of opportunity to avert full-scale civil war.
Syrian television showed a man pointing to the wreckage. "Is this freedom? This is the work of the Saudis," he said. Saudi Arabia 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 Qatari Prime Minister Sheikh Hamad bin Jassim al-Thani, who also says Syrian insurgents 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.
Qatar condemned the blasts in Damascus and called on all sides to stop the bloodshed in Syria.
The European Union also denounced the bombings as "pure terrorism", but said Annan's peace plan, backed by the EU, the United Nations and the Arab League, was still viable.
"It is the best option to try and ensure peace in Syria," Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, said in Brussels. "It is the best way forward."
Annan's plan calls for a ceasefire, political dialogue between the government and opposition, and unfettered access for humanitarian aid and journalists to Syria.
Western powers have shunned any Libya-style military intervention in Syria, while Russia and China have blocked any U.N. Security Council action against Damascus, although both have supported the U.N.-Arab League envoy's peace effort.
Russia condemned the bombings, accusing unspecified foreign countries of encouraging such violence and saying Moscow would not yield to pressure to change its stance on Syria.
"Some of our foreign partners are doing practical things so that the situation in Syria explodes in literal and figurative sense," state-run RIA quoted Foreign Minister Sergei Lavrov as saying in Beijing, without naming any countries.
Russia has been a strategic ally and major arms supplier to Syria during its four decades under Assad family rule. Moscow opposes Western calls for U.N. sanctions against Damascus and any foreign interference in Syria, saying there should be an end to violence by all sides and then dialogue without preconditions, such as Assad leaving power.
France, among Assad's sternest critics, said Annan's plan was the "last chance" to end the crisis. "The regime carries full responsibility for the horrors in Syria," the Foreign Ministry said. "By choosing a blind and brutal repression, the regime has entered a spiral of violence with no way out."
The U.S. embassy in Beirut called the double bombing "reprehensible and unacceptable" but reiterated Washington's demand that the Syrian government implement Annan's plan.
The United Nations says Syrian forces have killed 9,000 people during the revolt. Damascus blames foreign-backed "terrorists", saying they have killed 2,600 soldiers and police.
Samir Nashar, a member of the opposition Syrian National Council's executive board, said the government had stuck to violence and had not implemented any of Annan's six-point plan.
"We want international intervention to stop this policy of killing," he added, without saying what form it should take.
Nashar blamed the state for the bombings, saying they were intended to deter protesters and international monitors, an argument echoed by leaders of the rebel Free Syrian Army (FSA).
"These bombs are not the work of opposition fighters," said its chief, General Mustafa al-Sheikh, adding that the FSA lacked the capability to set off such big explosions.
Riad al-Asaad, the FSA's commander of operations, said the rebels were ready to resume attacks on government forces as soon as Annan announced that his initiative had failed.
In other violence, 10 rebels were killed overnight when tanks shelled the village of Ain Sheeb in the northwestern province of Idlib, opposition sources said. Tank fire also killed a civilian in the northwestern town of Ain Hamra.