Two suicide car bombers killed 55 people and wounded 372 in Damascus today, state media reported.
The blasts further undermine a ceasefire declared by international mediator Kofi Annan on 12 April.
Opposition leaders said Mr Annan's peace plan was dead, while Western powers insisted it remained the best way forward.
Mr Annan 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.
The White House and the United Nations also condemned the attacks.
Syria's foreign ministry said the attacks were a sign that the major Arab state was facing foreign-backed terrorism and urged the UN 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 practising and encouraging terrorism," the state news agency SANA quoted the ministry as saying in a letter to the UN body.
Reports from Syria cannot be independently verified due to restrictions on foreign media.
The near-simultaneous explosions hit the al-Qazaz district just before 8am, residents said.
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.
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 12 April truce accord. The toll did not include today's deaths.
