Syrian security forces fired tear gas at tens of thousands of protesters who were heading towards the centre of Syria's troubled city of Homs, the Syrian Observatory for Human Rights said.

"There are at least 70,000 protesters. They are marching towards the city centre and the security forces are trying to stop them. They are firing tear gas," the Observatory's Rami Abdelrahman told reporters.

The protesters seemed to have been emboldened by the visit of Arab monitors who were in Homs on a mission to assess whether Syria has ended a nine-months of crackdown on protests against Syrian President Bashar al-Assad.

Arab League monitors finished their first day of observation in Syria's flashpoint city of Homs today and will continue touring the area tomorrow, the Syrian television channel Dunia said.

Arab League peace monitors are on a mission to assess whether Syria has halted its nine-month crackdown on protests against President Bashar al-Assad. Homs, which has been at the centre of the protests, was one of their first stops.