Al Jazeera's Dorsa Jabbari, reporting from Beirut in neighbouring Lebanon, said demonstrations were taking place in 18 different provinces across Syria.

"One of the biggest demonstrations is near the capital, Damascus. Activists have called on all residents to try and reach the centre of their cities. In Damascus ... they are being met by security forces who have used tear gas, according to many of the residents nearby," she said.

"We know that large demonstrations are also under way in the city of Hama, [in] Homs, as well as [in] Idlib, where we've seen images of observers themselves along with the demonstrators in the midst of it all."