A car bomb hit Syria's second city Aleppo yesterday, a day after explosions killed 27 in Damascus, and security forces arrested and beat activists at a rare anti-Government protest in the centre of the capital.

The British-based opposition Syrian Observatory for Human Rights said at least three people had been killed and 25 wounded by the explosion close to a state security office in Aleppo.

Syria TV said the "terrorist" attack had been between two residential buildings in the al Suleimaniya district, behind a post office building.

It showed building fronts blasted open, masonry littering the road and blood all over a street corner.

Activists and the Government traded blame for the explosion, as they have over previous bombings. TV showed pictures of bloodied bodies and charred buildings from earlier blasts. "Their gift to us," said a caption, followed by a bloody handprint. "Their fingerprints are obvious."

But an activist in Aleppo from the opposition's local Revolutionary Council said the Government was behind the attack.

"These explosions are always done by the regime to discourage people from joining the revolution ... they want to make our uprising seem like a terrorist operation to the rest of the world, but it is not," said the activist, who called himself Marwan, by telephone to Beirut.

The opposition reported heavy raids by security forces and fighting with rebels in northern and southern Syrian provinces and suburbs of Damascus.

In the capital, as crowds gathered for memorials to victims of Saturday's car bombs, security forces broke up an opposition march of more than 200 people when protesters began shouting "the people want to topple the regime".

The phrase has echoed through the wave of Arab uprisings that began last year and has toppled autocratic rulers in Tunisia, Egypt, Libya and Yemen.

"They were walking through an area in central Damascus. At first they shouted slogans against violence and the police didn't do anything, but as soon as they started to call for regime change the police rushed in and started beating people with canes," said Rami Abdelrahman, from the Syrian Observatory for Human Rights.

The protest, which called for non-violent resistance to the Government, had been led by moderate opposition leaders previously tolerated by the Government because of their calls for dialogue and rejection of foreign intervention.

In central Damascus crowds of Syrians assembled at the sites of the two car bombings outside security bases on Saturday, and dozens of people waving Syrian flags gathered to pray for the victims.

"Whatever they want to do we won't be afraid," said one woman speaking to Syria TV.

"We won't be afraid, we are with President Assad ... those who don't love Damascus should leave."