Syrians braced for mass protests on Friday, a day after at least 55 people were killed in twin bombings that marked the deadliest attack since the start of the anti-government uprising 14 months ago.

The car bomb explosions in Damascus Thursday were among a string of attacks that have occurred since U.N. observers arrived in Syria to monitor a shaky cease-fire brokered by international envoy Kofi Annan in April.

The Syrian government on Friday urged the U.N. Security Council to take action to combat terrorism, in the wake Thursday's blasts.

State media said the government made the plea in letters to the Security Council and U.N. chief Ban Ki-moon. It said “escalating crimes” were proof that Syria is facing terrorist attacks led by groups receiving foreign support.

U.N. observers toured Damascus on Friday.

Observer mission spokesman Neeraj Singh said the number of international monitors and staff members taking part in the mission had grown to 150.

“You have the world coming together, the world community coming together to be with the people of Syria to see in what way we can help. The most important thing being that violence in all its forms has to stop.”

The head of Syria's main opposition group said al-Qaida-linked forces with ties to the Syrian government were responsible for Thursday's blasts.

At a Friday news conference in Tokyo, Burhan Ghalioun said the government was trying to sabotage the peace plan brokered by international envoy Kofi Annan.

There was no immediate claim of responsibility for the blasts, which further shredded the April 12 cease-fire declared by Mr. Annan.

U.S. Defense Secretary Leon Panetta told reporters U.S. intelligence indicates “an al-Qaida presence in Syria,” but said the extent of its activity is unclear.

The United Nations says more than 9,000 people have been killed in violence related to the anti-government uprising which erupted in March 2011.