A car bomb went off in Syria's second city of Aleppo Sunday, a day after explosions killed 27 in Damascus, and security forces arrested and beat activists at a rare protest in the heart of the capital.

The state news agency SANA said the attack by terrorists had killed two people and wounded 30 others, Reuters reported. But opposition activists said three died in the explosion, near a Political Security office and a church.

The semi-official news channel al-Ikhbariya said security forces had been tipped off about the bomb and had been evacuating people out of the area when it went off. It said the car had been filled with 200 kilograms of explosives.

Pictures on SANA's website showed building fronts blasted open and aid workers standing near piles of shattered masonry and bomb craters. Syria TV showed at least one street corner splattered in blood.

Local activist Mohammed Halabi told Reuters at least 15 ambulances and security cars rushed to the area after the blast.

The blast was extremely loud and even shook nearby areas, he said, speaking by telephone.

No group has claimed responsibility, but opposition supporters accused the Assad regime.

They want to make our uprising seem like a terrorist operation to the rest of the world, but it is not, an activist calling himself Marwan, told Reuters by telephone.

This is the regime's game. This is how they play their dirty tricks. They carry out these types of explosions from time to time to get more international support and compassion, Capt. Ammar al-Wawi of the rebel Free Syria Army told CNN. They are desperately trying to prove to the world that they are fighting against armed gangs, but the reality is they are the ones who are doing all the killings.

Residents told the British-based Syrian Observatory for Human Rights they saw bodies in the streets of Aleppo, Reuters reported. The Observatory said the blast hit near a state security office.

State news channel Syria TV said the terrorist explosion had been between two residential buildings in the al-Suleimaniya district of Aleppo, behind a post office building.

It showed building fronts blasted open, masonry littering the street and a blood-spattered street corner.

Mohammed Saeed, an Aleppo resident, told the AP a car bomb exploded near the Political Security Directorate in the city's central neighborhood of Suleimaniyeh. He said the neighborhood has a large Christian population.

Activists and the government traded blame for the explosion, as they have over previous bombings. After other similar attacks, U.S. officials suggested al Qaeda militants may be joining the conflict. Previous attacks have been claimed by shadowy extremist groups as revenge for the regime's bloody onslaught on Homs.

The TV channel showed pictures of bloody bodies and charred buildings from earlier blasts. Their gift to us, said a caption, followed by a bloody handprint. Their fingerprints are obvious.

In Damascus, 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, a refrain of all the Arab uprisings.

They were walking through an area in central Damascus, near SANA (the state news agency). 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.

Meanwhile, violence continued across the country Sunday.

At least 20 people were killed in clashes, including two children and seven soldiers from the Free Syria Army, the opposition Local Coordination Committees of Syria told CNN. The deaths included nine people killed in Idlib and four people killed in the Damascus suburbs, the network of opposition activists reported.

There were reports of several other casualties after shelling in the northeastern city of Deir Ezzor, according to the LCC.

Four Syrian soldiers were killed in fighting with defectors near the northern city of Jisr al-Shugur, the Syrian Observatory for Human Rights said.