A large explosion struck the Syrian city of Aleppo on Friday, close to the ruling Baath party's headquarters, the Syrian Observatory for Human Rights reported.

"A big explosion was heard near Al-Jabiri Square. It targeted an office of the Baath party, according to initial reports," the Britain-based watchdog said in a statement.

"The blast was powerful but we do not know what was the origin of the explosion," the Observatory's Rami Abdel Rahman added.

No casualties from the blast have yet been reported, although witnesses said a guard at the headquarters was shot dead by an unidentified gunman soon after the explosion.

It comes just hours after Syrian forces claimed to have foiled an attempted suicide bomber in the city. State television reported that the would-be bomber, whose car was laden with 1,200 kilograms of explosives, was killed in the operation.

Contradictory allegations

That earlier television report has not yet been independently verified and the news agency AFP quoted anti-regime activists as dismissing the report as a lie.

"It is not in the interests of the [rebel] Free Syrian Army to stage attacks on a Friday," Aleppo-based activist Mohammad al-Halabi said.

For months, Friday has been the day that activists have held weekly protests against the regime of Bashar al-Assad.

"The regime is trying to make people believe there is Al-Qaeda here, but there is no Al-Qaeda here," al-Halabi added.

The Syrian government accused al-Qaeda-linked groups of being behind twin-bombings in the capital Damascus on Thursday, as well as other attacks which have hit the city in recent months. Opposition activists, meanwhile, have accused the regime of staging bomb attacks in an attempt to discredit them.

Syrians out in force

At least 55 people were killed and 372 others were injured on Thursday in the deadliest attack to take place in Syria since the conflict erupted in March 2011. The Syrian Interior Ministry reported that suicide bombers had detonated two cars packed with explosives near a military intelligence building in Damascus.

Thousands of people staged protests in Aleppo and other cities across Syria on Friday to condemn the attacks.

According to opposition activists, protesters in the flashpoint city of Homs chanted: "The regime is killing the people of Syria," as they gathered after noon prayers.

ccp/ipj (AFP, Reuters, dpa)