Syrian state-run television reports that at least 55 people were killed and more than 370 injured in a double bomb attack in the capital, Damascus.
The Interior Ministry said the blasts on May 10, one of which targeted a Syrian military intelligence building, were caused by suicide car bombers.
A spokesman for the opposition Syrian National Council blamed the regime of President Bashar al-Assad, alleging the regime staged the explosions to intimidate United Nations monitors and convince the international community it is battling "terrorists."
The attacks were condemned by the UN Security council, as well as the UN Secretary-General Ban Ki-moon, who "strongly condemned" the attacks and called on all sides to cease armed violence and distance themselves from "indiscriminate bombings and other terrorist attacks."
UN-Arab League envoy Kofi Annan also condemned the blasts and called for a cease-fire agreed last month to be observed.
The head of the UN observer mission, Major General Robert Mood, visited the scene of the carnage and condemned the attacks.
Television footage has shown dozens of destroyed vehicles and charred bodies.
Reports say at least one of the blasts targeted a Syrian military intelligence base.
There has been no claim of responsibility.
Mood urged "everyone within and outside Syria to help stop the violence" between government forces and opposition supporters.
Damascus has been the target of numerous bomb attacks in recent months.
On May 9, a roadside bomb narrowly missed a convoy from the United Nations observer mission in Syria. The bomb exploded outside the southern city of Daraa, near Syrian military escorts for Mood and 11 other UN monitors.
The UN has authorized 300 monitors in Syria, and 70 have been deployed so far.
UN-Arab League envoy Kofi Annan warned on May 8 that Syria is on the verge of "civil war."
Based on reporting by AFP and dpa