captured members of Gaddafi's convoy in Sirte," his home town, said Peter Bouckaert, emergencies director at Human Rights Watch (HRW).
"It also looks as if they took Mutassim Gaddafi, who had been wounded, to (the port city of) Misrata and killed him there," he said of Gaddafi's son.
"Our findings call into question the assertion by Libyan authorities that Muammar Gaddafi was killed in crossfire, and not after his capture," Bouckaert said of a Human Rights Watch report documenting the executions.
The 50-page report, "Death of a Dictator: Bloody Vengeance in Sirte," also details the final hours of Gaddafi's life and the circumstances under which he was killed on the basis of witness testimony and mobile phone footage.
HRW said its evidence suggests that Misrata-based militias captured and disarmed members of Gaddafi's convoy and, after bringing them under control, subjected them to brutal beatings.
"They then executed at least 66 captured members of the convoy at the nearby Mahari Hotel," said the report, adding that some still had their hands bound behind their backs.
That finding matches reports by an AFP correspondent who visited Sirte last October and documented the discovery of between 65 and 70 rotting bodies on the lawn of Al-Mahari Hotel, many with a bullet in the head.
HRW said it collected mobile phone video clips taken by anti-Gaddafi fighters that show a large group of captured convoy members being cursed and abused.
The watchdog added that it used hospital morgue photos "to establish that at least 17 of the detainees visible in the phone video were later executed at the Mahari Hotel."
To document what happened on October 20, 2011, HRW said it had interviewed officers in opposition militias who were at the scene and surviving members of the Gaddafi convoy in hospital, in custody or at home.
"These killings constitute the largest documented execution of detainees by anti-Gaddafi forces during the eight-month conflict in Libya," the report said.
The watchdog stressed that evidence regarding the deaths of Moamer and Mutassim Gaddafi also calls into question the official account given by the Libyan authorities, which said the two "died during fierce crossfire."
Video footage shows that the dictator was captured alive but bleeding heavily from a head wound. In the footage, he is severely beaten by rebels and appears to be stabbed with a bayonet in his buttocks.
"He appears lifeless" by the time he is filmed being loaded into an ambulance half-naked, the organisation said.
On the basis of separate footage, the watchdog said Mutassim was caught alive and taken to Misrata where he is seen smoking and having a "hostile conversation" with his captors.
"By the evening, his dead body, with a new wound on his throat that was not visible in the prior video footage, was being publicly displayed in Misrata," which had suffered a brutal siege by Gaddafi's forces, the organisation said.
HRW said it presented its findings to transitional officials immediately after the killings and has repeatedly urged the new authorities to carry out a full investigation into these killings which amount to a war crime.
It added that it had not seen "any evidence" of any inquiry under way.
"In case after case we investigated, the individuals had been videotaped alive by the opposition fighters who held them, and then found dead hours later," Bouckaert said.
"One of Libya's greatest challenges is to bring its well-armed militias under control and end their abuses.
"A good first step would be to investigate the mass executions of October 20, 2011, the most serious abuse by opposition forces documented so far," Bouckaert said.