"We found 53 decomposing bodies, apparently Kadhafi supporters, at an abandoned hotel in Sirte," said Peter Bouckaert of Human Rights Watch (HRW), who investigated the killings.
"Some had their hands bound behind their backs when they were shot," he added.
HRW's investigator found the bodies on Sunday at the Hotel Mahari in District 2 of Sirte, an "area of the city that was under the control of anti-Kadhafi fighters from Misrata before the killings took place."
"The bodies were clustered together, apparently where they had been killed, on the grass in the sea-view garden of the hotel," HRW said in a statement.
An AFP reporter at the weekend found 60 corpses on the lawn of the Al-Mahari hospital and noted that many of the victims had been killed execution-style, a bullet to the head. Some had been bound hand and foot.
NTC fighters told AFP at the time that the hospital had been used as a makeshift prison by Kadhafi's men who, they charged, carried out the executions before abandoning the place.
Similar atrocities were carried out in Tripoli, with at least 50 charred skeletons, apparently prisoners executed by Kadhafi's fleeing forces, being found after the capital fell to National Transitional Council (NTC) fighters late August.
Human Rights Watch said the Sirte killings likely took place a week before the bodies were discovered.
Citing testimony from Sirte residents who played a role in burying and identifying the dead, it concluded that "most" of those killed were likely to be residents of the Mediterranean city and "some" of them Kadhafi supporters.
The watchdog implicated several brigades from Misrata, Libya's third city, who "apparently" held the hotel from before the time of the killings until October 20 when the fighting stopped.
Graffiti on the walls showed the names of five Misrata-based groups, it said.
"If the NTC fails to investigate this crime it will signal that those who fought against Kadhafi can do anything without fear of prosecution."
HRW said that at a separate site in Sirte the decaying bodies of 10 people, also seemingly executed, were found near a water reservoir but the rights group was unable to determine their identities.
"Medical officials in Sirte told Human Rights Watch that pro-Kadhafi forces had carried out executions in the city," the New-York based watchdog said.
The allegations of executions carried out by fighters who took part in the NTC's offensive in Sirte come just days after the unexplained deaths of Libya's ex-leader and his son Mutassim Kadhafi in custody of Misrata fighters.
HRW said it also found the "remains of at least 95 people" who apparently died the day Kadhafi was captured and that at least six of the dead appeared "to have been executed at the site with gunshot wounds to the head and body."
"This latest massacre seems part of a trend of killings, looting, and other abuses committed by armed anti-Kadhafi fighters who consider themselves above the law," Bouckaert said.
"It is imperative that the transitional authorities take action to rein in these groups."