The report, based on a one-year investigation by a three-member commission, also found forces loyal to the former dictator had carried out mass executions and tortured opponents of the regime in what constituted war crimes and crimes against humanity, The Washington Post reported.
In addition, the report said, NATO forces may have accidentally killed dozens of citizens in five airstrikes that killed 60 civilians and injured 55 others.
The 220-page report "concluded that international crimes, specifically crimes against humanity and war crimes, were committed by Gadhafi's force in Libya. Acts of murder, enforced disappearance and torture were perpetrated within the context of a widespread or systematic attack against the civilian population."
The New York Times said the report also found anti-Gadhafi forces "committed serious violations," including breaches of international rights law.
Through January, the report said, militia members conducted mass arrests of former soldiers, police officers, suspected mercenaries and others believed to be Gadhafi loyalists. Some revenge attacks by militias have continued, the report said.
The commission called on the government to investigate militia killings.
"Libyan authorities can break with the Gadhafi legacy by enforcing the law equally, investigating all abuses -- irrespective of the perpetrator," the report said.
The report also said Libyan authorities have not provided access to an autopsy report for Gadhafi nor information on circumstances of the death of his son Mutassium. Both had died while in custody of anti-regime forces.