At least 25 people were killed when a suicide bomber detonated a device in Damascus, according to the official Syrian state news agency.
Scores more were wounded in the attack, which appeared to have targeted a police bus in the centre of the Syrian capital.
It happened just two days before an Arab League committee were due to discuss the initial report of their observers, sent to monitor the popular uprising against President Bashar al-Assad.
The meeting in Cairo might decide to refer Syria to the UN Security Council, which could lay the ground for some form of international action to try to end the bloodshed.
The United Nations estimates that 5,000 people have been killed during the uprising. First hand reporting is virtually impossible, as most independent journalists have been kept out of Syria.