Syrian security forces killed four demonstrators on Friday in the southern city of Deraa as they took part in a peaceful protest demanding political freedom and an end to corruption in Syria, a human rights activist said.

Akram al-Jawabra, Houssam Abdelwali Ayash, Ayham al-Harri and a member of the Abu Aoun family were among several thousands who where chanting "God, Syria, Freedom" and anti-corruption slogans, accusing the family of President Bashar Assad of corruption, when they were shot dead by security forces who were reinforced with troops flown in by helicopters, the activist said.

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"Hundreds of protesters were wounded and many were snatched by the security force from the hospital where they had been taken and removed to an unknown location," he added.

Plain-clothes Syrian police broke up a protest after Friday prayers at the main mosque in central Damascus, dragging away at least two activists, AFP reporters witnessed.

"There is no God but God," a crowd inside the men's section of the Omayyed Mosque chanted in crescendo after Friday prayers at noon.

Dozens of security agents who had gathered outside the mosque during the prayers, pulled out batons as soon as the chants broke out and detained at least two people, beating one one who resisted and kicking him in the nose.

At least 200 people immediately rallied in a square outside the mosque, chanting support for President Bashar al-Assad and waving Syrian flags. Some carried portraits of his late father and predecessor Hafez al-Assad.

'No mass discontent'

Terrified families could be seen fleeing the square, with many children in tears.

It was unclear what sparked the chaos, but a Facebook group The Syrian Revolution 2011 had called for demonstrations after Friday prayers during a "Day of Dignity."

A video posted on the group's Facebook page showed a crowd of men inside the mosque chanting "there is no God but God," while a few calls of "freedom" could be heard before being drowned out.

Another video showed one man being dragged out of the mosque by other men who had attended the prayers.

Demonstration in Deraa

The Facebook group also posted a video of a rally outside a landmark mosque in the city of Homs, 150 kilometres (about 100 miles) north of Damascus, where dozens of protesters marched, chanting "God, Syria, freedom."

Another video aired on Facebook showed what it described as demonstrators in Deraa shouting slogans earlier in the day against Syrian tycoon Rami Makhlouf, a cousin of President Assad, who owns several large businesses.

"Makhlouf you thief," shouted dozens of demonstrators marching in the streets.

State television said some "infiltrators" in the town of Deraa caused "chaos and riots" and smashed cars and some property before they were chased off by riot police. It said a similar demonstration in the coastal town of Banyas was dispersed without incident.

Two other videos, allegedly from the coastal city of Banias, showed crowds, one with at least 100 people, chanting for freedom.

According to websites affiliated with the Syrian opposition, the number of people killed and wounded in Deraa is much higher than what was initially reported. "According to information obtained this evening, the death toll stands at 54," one website claimed.

It was further reported that Syrian families posted messages online calling on their sons who serve in the army to disobey their commanders' orders to open fire on demonstrators.

According to another report, a soccer game held in the remote eastern Syrian city of Dir a-Zur turned into an anti-government protest.

Members of Syria's ruling hierarchy have indicated that they believe they are immune from the uprisings sweeping the Arab world, but small nonviolent protests this week have challenged their authority for the first time in decades.

On Wednesday plain-clothed security forces wielding batons dispersed 150 demonstrators in central Damascus who had gathered outside the Interior Ministry to demand the release of political prisoners.

Assad, who succeeded his father 11 years ago, is also head of the Baath party, which has been in power since 1963, banning opposition and imposing the emergency law still in force.

He said in an interview published in January that Syria's ruling hierarchy was "very closely linked to the beliefs of the people" and that there was no mass discontent against the state.

New York-based Human Rights Watch has said Syria's authorities were among the worst violators of human rights in 2010, jailing lawyers, torturing opponents and using violence to repress ethnic Kurds.

Bashar's father, Hafez al-Assad, sent troops into the city of Hama in 1982 to finish off the armed wing of the Muslim Brotherhood. Around 30,000 people were killed and much of the old quarter of the city was razed to the ground.

Reuters, AP, AFP contributed to the report