Assad, facing the greatest challenge to 40 years of Baath Party rule, has sought to crush demonstrations that broke out in March. But although rights groups say some 1,400 civilians have been killed, the protests have grown.

"These are the biggest demonstrations so far. It is a clear challenge to the authorities, especially when we see all these numbers coming out from Damascus for the first time," said Rami Abdelrahman, head of the Syrian Observatory for Human Rights.