The rights group said it found the bodies clustered together at Hotel Mahari in Sirte on Sunday. About 20 residents were putting the bodies in body bags to prepare them for burial when Human Rights Watch found them.
"We found 53 decomposing bodies, apparently (Gadhafi) supporters, at an abandoned hotel in Sirte, and some had their hands bound behind their backs when they were shot," said Peter Bouckaert, emergencies director of Human Rights Watch.
"This requires the immediate attention of the Libyan authorities to investigate what happened and hold accountable those responsible."
Residents told Human Rights Watch investigators they found the bodies last week after the fighting in Sirte stopped and they returned home.
They identified some of the deceased as Sirte residents and Gadhafi supporters.
Officials with the National Transitional Council, Libya's new leadership, were not immediately available for comment.
A NATO official noted that Secretary-General Anders Fogh Rasmussen has repeatedly applauded the Libyan council for saying it is committed to human rights, the rule of law and reconciliation.
"What's important for us is to see that they're doing their utmost to get this message out about restraint and pulling the country together," the official said, speaking on condition of anonymity because the official is not authorized to speak by name on the record. NATO has "no way of verifying" the Human Rights Watch report and will not comment on it specifically, the official said.
The human rights group's report comes amid growing concerns about extrajudicial killings under Libya's new leadership.
U.S. Secretary of State Hillary Clinton told CNN Sunday that the United States supports calls by the United Nations and by Libya's National Transitional Council for an independent investigation into the death of Gadhafi. He was killed last week by a gunshot wound to the head.
Mahmoud Jibril, executive chairman of the National Transitional Council's executive board, has said Gadhafi's right arm was wounded when a gunbattle erupted between the fighters and Gadhafi loyalists as his captors attempted to load him into a vehicle.
More shooting erupted as the vehicle drove away, and Gadhafi was shot in the head, dying moments before arriving at a hospital in Misrata, Jibril said, citing the city's coroner.
Human Rights Watch, in its statement Monday, complained of the "still unexplained deaths" of Gadhafi and his son Mutassim while in the custody of fighters.
"At the site where Moammar Gadhafi was captured, Human Rights Watch found the remains of at least 95 people who had apparently died that day. The vast majority had apparently died in the fighting and NATO strikes prior to Gadhafi's capture, but between six and ten of the dead appear to have been executed at the site with gunshot wounds to the head and body," the group said.
A senior NATO official said Gadhafi's death came after he survived a NATO airstrike on a convoy in the Sirte area.
Mustafa Abdul Jalil, chairman of the NTC, said Monday in Benghazi that the council has established a committee to deal with Gadhafi's body. "The procedure will follow a fatwa made by the Islamic fatwa society," he said.
Lt. Gen. Charles Bouchard, commander of the NATO military operation, said at a news conference Monday, "We saw a convoy, and in fact we had no idea that Gadhafi was on board." It was a surprise that Gadhafi was in the area, Bouchard said. The convoy was carrying weaponry, and seemed to present "a clear threat to the population," he said.
Human Rights Watch, in its statement Monday, also reported other instances of bodies found recently in Sirte.
At a separate site in the city, "Human Rights Watch saw the badly decomposed bodies of 10 people who had apparently also been executed," the group said. "The bodies had been dumped in a water reservoir in District 2 of the city. The identity of the victims was unknown, and it was not possible to establish whether Gadhafi forces or anti-Gadhafi fighters were responsible."
Sirte, Gadhafi's hometown, was one of the last cities to fall before the National Transitional Council declared the country liberated Sunday following his 42-year rule.
However, anti-Gadhafi fighters from Misrata had controlled the area of Sirte where the hotel is located since early October, Human Rights Watch said, citing witnesses.
On the entrance and walls of the hotel, the group said, it saw the names of several brigades from Misrata.
Based on the condition of the bodies, the group's investigators determined the 53 had been killed between October 14 and 19.
"The evidence suggests that some of the victims were shot while being held as prisoners, when that part of Sirte was controlled by anti-Gadhafi brigades who appear to act outside the control of the National Transitional Council," Bouckaert said.
"If the NTC fails to investigate this crime it will signal that those who fought against Gadhafi can do anything without fear of prosecution."
In addition, medical officials in Sirte told the group that pro-Gadhafi forces had carried out killings in the city and that they had found 23 bound bodies between October 15 and 20.
