The 50-page report by Human Rights Watch also calls into question assertions by Libyan authorities that Gadhafi was killed by crossfire during a gun battle in October 2011.
HRW's report details Gadhafi's final hours and how he died. It presents evidence that militias based in Misurata captured and disarmed members of the despot's convoy then beat them before killing them.
The report, titled "Death of a Dictator: Bloody Vengeance in Sirte," claims at least 66 members of the convoy were executed at the Mahari Hotel in Sirte.
Gadhafi's wounded son Mutassim was taken by militia members to Misurata, where he was killed, the report says.
Evidence cited by the human rights group included a video clip made by opposition militias on a cellphone that shows a large number of people captured in the convoy being cursed and abused while in detention.
HRW said it used photos from hospital morgues to verify that at least 17 of the people shown detained on the video were later executed at the hotel.
The killing of captured combatants is a war crime. HRW said Libyan authorities have the responsibility to investigate such violations.
The rights organization said the killings of the convoy members "constitute the largest documented execution of detainees by anti-Gadhafi forces during the eight-month Libyan conflict.