At least three protesters have been shot dead in the south Syrian city of Deraa as security forces clamped down on a protest rally.
They were killed by security forces as protesters demanded political freedom and an end to corruption, eyewitnesses and activists told foreign media.
President Bashar al-Assad, whose Baath party has dominated politics for nearly 50 years, tolerates no dissent.
Washington strongly condemned the use of force against demonstrators.
White House national security council spokesman Tommy Vietor said the US was calling on the Syrian government to "allow demonstrations to take place peacefully".
"Those responsible for today's violence must be held accountable," he added.
UN Secretary General Ban Ki-moon said the "use of lethal force against peaceful demonstrators and their arbitrary arrests" were "unacceptable", according to a spokesman.
"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," he added.
Syria's state news agency Sana said violence and "acts of sabotage" had broken out at a demonstration in Deraa on Friday, prompting security forces to intervene.
It accused "infiltrators" of seeking to "provoke chaos through acts of violence which resulted in damage to private and public property".
Two of the dead people were named by witnesses as Hussam Abdel Wali Ayyash and Akram Jawabreh.
They had been among "several thousand" demonstrators chanting "God, Syria, Freedom" and anti-corruption slogans, accusing the president's family of corruption, a resident told Reuters news agency.
Security forces, the Reuters source added, were reinforced with troops flown in by helicopters.
An unnamed human rights activist in Deraa, who was contacted by AFP news agency, said security forces had fired live bullets at the protesters and "hundreds" of protesters had also been wounded.
He told AFP that "many" of the wounded had been "snatched by security forces" from hospital and moved to unknown locations.
Mazen Darwish, a prominent Syrian activist in Damascus, was quoted by the Associated Press news agency as saying that at least five people were shot and killed. He was citing eyewitnesses and hospital officials at the scene.