Syria's security forces have committed systematic "crimes against humanity" in their crackdown on anti-government protesters, a UN report says.
The study by an independent panel says civilians - including children - have been murdered, tortured and sexually assaulted.
Syria says it is fighting armed gangs. More than 3,500 people have reportedly died in the violence since March.
Meanwhile, Syria condemned the Arab League's imposition of sanctions.
At a news conference, Syrian Foreign Minister Walid al-Mouallem described the league's move on Sunday as a declaration of "economic war" on Damascus, and said the country had already withdrawn 95% of its assets from Arab countries
The sanctions include an asset freeze and an embargo on investments.
Syria's state TV showed footage of a huge rally in Damascus of supporters of President Bashar al-Assad, who denounced the sanctions.
In the latest violence, at least 23 people were killed across Syria on Sunday, activists say.
The claims cannot be independently verified as most foreign media are banned from entering Syria.
The three-member UN commission released its 39-page report at a news conference in Geneva on Monday.
"The commission is gravely concerned that crimes against humanity have been committed in different locations" in Syria, the document says.
The panel says it interviewed 223 victims, witnesses, and army defectors to investigate human rights violations from the end of September until mid-November 2011.
However, the investigation team members say they were denied entry into Syria itself.
The report contains allegations of abuse, summary executions and sexual violence against civilians detained during protests.
Some of those interviewed told of "shoot-to-kill" orders to crush demonstrators.
One army defector recounted how he had shot into the air instead and was later arrested, beaten and tortured.
The report also accuses the security forces of killing at least 256 children during the unrest.
"The sheer scale and consistent pattern of attacks by military and security forces on civilians and civilian neighbourhoods and the widespread destruction of property could only be possible with the approval or complicity of the state," the document says.
The commission - which was set up by the UN Human Rights Council - urges the Syrian government to end the violence immediately and punish those responsible.
Syria has so far not co-operated with any UN requests, the BBC's Imogen Foulkes in Geneva reports.