UN Secretary General Ban Ki-moon has described Syria's harsh crackdown on protests and use of deadly force against demonstrators as "unacceptable".
"The use of lethal force against peaceful demonstrators and their arbitrary arrests are unacceptable," Ban said on Friday, according his spokesman Martin Nesirky.
Syrian security forces killed three protesters and wounded hundreds in the southern city of Daraa on Friday, rights groups said, as demonstrations erupted across the country in the first major show of discontent under Bashar al-Assad's rule.
After two or three hours of clashes, the city was quieter at nightfall, with a heavy security presence.
Stones lay on the road where protesters and security forces clashed, and people said they were not allowed to visit the wounded in hospital.
Ban "urges the Syrian authorities to refrain from violence and to abide by their international commitments regarding human rights which guarantee the freedom of opinion and expression, including the freedom of the press and the right to peaceful assembly."
Ban "believes that, as elsewhere, it is the responsibility of the government in Syria to listen to the legitimate aspirations of the people and address them through inclusive political dialogue and genuine reforms, not repression," Nesirky said.
The statement came as the United States said it "strongly condemns the violence that has taken place in Syria," and called on the Syrian government "to allow demonstrations to take place peacefully," said National Security Council spokesman Tommy Vietor.
Protests 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 and rights groups reported that at least three people had been killed by security forces in the southern city of Deraa, where anti-government demonstrators had gathered after Friday 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.
'Acts of sabotage'
Witnesses said security forces were reinforced with troops who landed in the city's football stadium in helicopters.
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.