More than 20 people were killed on Monday in blasts targeting security buildings in the city of Idlib, northwest Syria, the Syrian Observatory for Human Rights said.
The majority of those killed were members of the security forces, the Britain-based group said.
Syrian state television put the death toll at eight, among them civilians, and said scores were also injured in two blasts that took place in Idlib's Hananu Square, on Carlton Street.
It said "terrorists" were behind the attacks.
It showed blood stains on the ground, and groups of angry people denouncing the violence and expressing support for the regime of President Bashar al-Assad.
"Is this the freedom they want?" shouted one man, standing near a woman who was carrying a child with blood running down his forehead.
One apartment block appeared in ruins and cars nearby were flattened by the force of the explosion.
A powerful blast, probably a car bomb, was also reported near the capital Damascus, causing casualties, the Observatory added.
"A strong explosion shook the suburbs of Qudsiya and it appears it was a car bomb," it said. "Initial reports indicate there are casualties."
Overnight, a rocket-propelled grenade hit the Central Bank in the capital, state media reported and on Friday a suicide car bomb in the heart of the capital also killed 11 people.
Anti-regime activists have accused the government of being behind the series of explosions, while authorities say "terrorists" are responsible.
"We confirm that these tricks no longer fool anyone, especially given the fact that the regime has resorted to these escalations every time there is political movement at the Arab, regional, or international level to find a political solution to the crisis in which the regime kills its people who are demanding freedom," said the activist, Local Coordination Committees.
The violence is taking place despite a UN-backed ceasefire that came into effect on April 12 but has failed to fully take hold.
Veteran peacekeeper Major General Robert Mood urged all sides on Sunday to abide by the ceasefire as he arrived in Damascus to take command of the UN military observer mission overseeing the truce.
The peace plan brokered by UN-Arab League envoy Kofi Annan calls for a commitment to stop all armed violence, a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate and the release of arbitrarily detained people.