At least 26 people have died in a suicide bombing in Damascus, state media in Syria has reported.
45 others - mostly civilians and some security personnel - were wounded in the blast, which state media blamed on "terrorists".
It said the "powerful explosion" happened in the historic Midan quarter in the heart of the capital.
The attack took place in a heavily populated neighbourhood near a school.
Gruesome footage was broadcast of rescuers gathering body parts in the streets.
Angry residents were at the scene shouting and denouncing the bombing as the work of "terrorists".
The Local Coordination Committees, an opposition activist group which has organised anti-regime protests since March, said in a statement sent to AFP in Nicosia that another explosion was heard in the Damascus suburb of Tal.
A team of Arab League observers has been in Syria since late December trying to assess whether President Bashar al-Assad's regime is complying with a peace accord aimed at ending its deadly crackdown on dissent.
The "Syrian Revolution 2011" group, one of the driving forces on the internet behind the uprising, had called on Facebook for demonstrations on Friday to urge the "internationalisation of our cause."
The appeal for people to take to the streets following the main weekly Muslim prayers comes ahead of an Arab League meeting this Sunday to discuss the mission which has come in for scathing criticism this week.
The head of the rebel Free Syrian Army has called on the Arab League to admit the mission has been a failure and urged the bloc to seek UN help to end the bloodshed.
The UN estimates that more than 5,000 people have been killed in the crisis since March, and the Syrian Observatory for Human Rights said regime forces killed another 17 civilians yesterday alone.
The government says armed "terrorists" have killed 2,000 members of the security forces in recent months.
