A bomb has exploded in Syria's second-largest city of Aleppo, one day after a double car bombing in the capital, Damascus, killed 27 people and wounded more than 100.

Syria's state news agency says that Sunday's explosion in Aleppo happened between two residential buildings. It says the attack was the work of “terrorists” whom the Syrian government blames for a year-long opposition uprising against President Bashar al-Assad.

The British-based group Syrian Observatory for Human Rights says the explosion killed three people and wounded about 25 and was the result of a car bomb near a security building. Details of the incident could not be independently confirmed.

In Saturday's attacks in Damascus, two car bombs exploded minutes apart near a police security building and an intelligence center. Crowds of Syrians gathered Sunday at the blast site for a prayer vigil. The Syrian government and opposition groups blamed each other for the bombings.

Several hundred opposition activists also gathered in central Damascus on Sunday for a rare march to mark the first anniversary of nationwide protests demanding greater political freedoms. But, Syrian security forces broke up the rally and detained several prominent opposition figures, including Mohammed Sayid and Fayiz Sara.

Syrian activists say government troops also carried out operations to block protests in the opposition hubs of Idlib, Deir Al-Zour and Daraa. They say rebels in Daraa blew up a bridge to prevent the military from bringing reinforcements to the area.

A group of U.N.-backed experts is due to arrive in Syria on Monday for talks with the government on the possible deployment of international monitors to try to end the country's year-long unrest. The experts are under the direction of Kofi Annan, the U.N.-Arab League joint special envoy for Syria.

Mr. Annan visited Damascus last week and urged Syrian President Assad to agree to an immediate end to military operations and a dialogue with the opposition.

The United Nations says at least 8,000 people have been killed in the Assad government's violent crackdown on the revolt, which began with peaceful protests and became increasingly militarized as army defectors attacked pro-Assad troops who assaulted civilians.