Human rights activists said at least 14 people were killed by security forces.

People took the streets in the capital, Damascus, and in several cities and towns, in what the opposition described as the biggest demonstrations yet.

US Secretary of State Hillary Clinton has warned that time is running out for Syria's government to usher in reforms.

Speaking during a visit to Lithuania, she said President Bashar al-Assad would face more organised resistance to his 10-year rule unless the country saw "a genuine transition to democracy".

Human rights groups say more than 1,350 civilians and 350 security personnel have been killed since protests began in mid-March.

The government has blamed the protests on armed gangs.

Mr Assad has promised a national dialogue on political reforms, but many protesters are demanding that he stand down immediately.