Protests have erupted in at least three towns across Syria in the most serious case of unrest in decades for a country that has been ruled with strict emergency laws for almost half a century.

Witnesses reported that at least three people had been killed by security forces in the southern city of Deraa on Friday, where anti-government demonstrators had gathered after midday prayers.

The three were reportedly among several thousand people in the city who chanted "God, Syria, Freedom" while accusing president Bashar al-Assad's family of corruption.

Witnesses said security forces were reinforced with troops who landed in the city's football stadium in helicopters.

"The confrontations are ongoing. They are heavy," a witness told the Reuters news agency.

Hundreds of protesters were reported to have been injured.

'Acts of sabotage'

The state news agency SANA said "acts of sabotage" had broken out at the protest, which prompted the security forces to intervene.

"Infiltrators took advantage of a gathering of citizens near the Omari Mosque in the city of Deraa on Friday afternoon to provoke chaos through acts of violence which resulted in damage to private and public property," the agency reported.

"The infiltrators also set cars and shops on fire, which obliged security forces to intervene in order to protect citizens and property. They were also attacked by the infiltrators before the latter dispersed."

The violence came after a 200-strong protest in the capital Damascus was forcefully broken up by baton-wielding plain-clothes Syrian police, witnesses said.

A video on the Facebook group The Syrian Revolution 2011, which earlier called for protests to mark Friday as a "day of dignity", showed a man being dragged out of the Omayyed mosque in central Damascus.

Soon after the protest there was dispersed, a crowd of government supporters appeared in the square outside the mosque, carrying pictures of al-Assad and his father, Hafez al-Assad.

Other videos which appeared online purportedly show water cannon being used on crowds of protesters in the coastal town of Banyas, and several thousand men gathering in the city of Homs.

Source: Al Jazeera and agencies