At least 19 people have been killed after Syrian security forces opened fire on demonstrators during some of the largest protests to date against President Bashar al-Assad's government.

Tens of thousands of demonstrators rallied across the country, where some waved large Syrian flags and chanted anti-government slogans.

Witnesses and activists say police responded with live ammunition and tear gas at some rallies, as the government continues its crackdown on dissent.

They say most deaths took place in the capital, Damascus, and its suburbs, but protesters were also shot dead in the southern city of Daraa, the central city of Homs and the northwestern Idlib province.

Meanwhile, the state-run SANA news agency says pro-government rallies took place in several cities.

Activists had urged citizens to protest against the imprisonment of hundreds of people in the government's crackdown on pro-democracy protesters.

The call to protest comes a day after Syrian rights activists said government forces had killed six people and arrested more than 20 others across the nation since Wednesday.

There was no independent confirmation of the casualties. Syria does not allow foreign media into flashpoint areas.

Syrian President Bashar al-Assad has been trying to crush a four-month old opposition uprising against his 11-year autocratic rule.

He also has made promises of political reforms, but opposition groups have dismissed the offers and demanded an end to the deadly crackdown on their movement.