A powerful explosion rocked Aleppo, Syria's most populous city, late Friday, killing a guard at an office of President Bashar al-Assad's ruling Baath Party, opposition groups reported.

The cause of the blast was not immediately known. Heavy gunfire was heard in its aftermath, the UK-based Syrian Observatory for Human Rights reported.

Earlier, Syrian authorities foiled a suicide bombing in the city, state media said.

The action comes a day after suicide attackers killed dozens in the capital of Damascus, a strike that heightened tensions in a country caught in the grip of a popular uprising.

Authorities "intercepted a stolen booby-trapped minibus" that an attacker tried to detonate in Aleppo's densely populated neighborhood of al-Shaar, the state-run Syrian Arab News Agency reported.

A SANA correspondent quoted a source in Aleppo province as saying that "competent authorities intercepted the terrorist after he hit two policemen, then he blew himself up with an explosive belt, killing himself," the news agency said. The source said that authorities searching the minibus found four tanks with "a big quantity of explosives."

An initial state TV report said the forces killed the bomber before he detonated his vehicle carrying 1,200 kilograms, or 2,645 pounds of explosives.

Damascus and Aleppo have been the scene of a flurry of attacks in recent months. Aleppo, a commercial center and long a bastion of support for al-Assad, had been largely spared in Syria's 14 months of bloody uprising. But recent protests and violence there could signal a significant shift.

Elsewhere Friday, a number of explosions were reported in cities across the country, including four in Daraa and several in Hama, according to the Local Coordination Committees of Syria, an opposition group.

At least 20 people, including the Aleppo security guard, were killed across the country, the Syrian Observatory for Human Rights said.