The U.N. Human Rights Council set up the commission in August. It presented its first report Monday at a news conference in Geneva.
Investigators gathered evidence from at least 223 victims and witnesses from the end of September through mid-November. The commission concluded that the army and security forces seriously breached international law.
"The commission has concluded based on its findings that members of the Syrian army and security force have committed crimes against humanity in their repression of a largely civilian population in the context of a peaceful protest movement," said the commission's chairman. "These crimes include murder, torture, rape and other forms of sexual violence ..."
The commission found that human rights violations happened with the consent of Syria's highest ranking officials.