A United Nations commission of inquiry says security forces in Syria have committed murder, torture and rape and the government of President Bashar al-Assad is responsible.
The panel was set up by the UN Human Rights Council in Geneva. It reached its conclusions after interviewing 223 witnesses, including defectors.
However, investigators say they were denied entry into Syria itself.
The commission is gravely concerned that crimes against humanity have been committed in different locations in Syria, the commission's report says.
More than 3500 people have reportedly been killed since March.
The panel on Monday called on Syria to halt violations of human rights, release prisoners and allow the media, aid workers and rights monitors access to the country.
Meanwhile foreign minister Walid al-Moallem says the Arab League's decision on Sunday to impose sanctions on Syria closes the window to resolving the crisis.
He says the League has refused to acknowledge the existence of an armed insurgency in Syria.
Demonstration in Damascus
State TV showed footage of a huge rally in Damascus of supporters of President Bashar al-Assad, who denounced the sanctions.
AFP reports they waved Syrian flags and chanted nationalist songs.
