11:26AM BST 24 Oct 2011
"We found 53 decomposing bodies, apparently Gaddafi 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.
In a statement, the group said: "If the NTC fails to investigate this crime it will signal that those who fought against Gaddafi can do anything without fear of prosecution."
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-Gaddafi fighters from Misurata 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 Gaddafi'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 Gaddafi'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 Gaddafi supporters.
The watchdog implicated several brigades from Misurata, 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 Misurata-based groups, it said.
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-Gaddafi 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 in custody of Misurata fighters.
HRW said it also found the "remains of at least 95 people" who apparently died the day Gaddafi 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-Gaddafi fighters who consider themselves above the law," Mr Bouckaert said.
"It is imperative that the transitional authorities take action to rein in these groups."
He spoke as the doctor who performed an autopsy on Gaddafi said the fallen dictator had been "killed by bullets" after his capture Thursday in Sirte.
But Doctor Othman el-Zentani, who examined Gaddafi's body, said: "My autopsy report is not finished."
Controversy has raged over the circumstances of Gaddafi's death after he was taken alive during the fall of his hometown Sirte. The new Libyan authorities have insisted the former dictator died as a result of crossfire while many sources spoke of a summary execution.
Libya's new leaders began Monday the tough task of forging an interim government uniting the nation's disparate political forces after 42 years of Gaddafi's iron-fisted rule, promising a system of Islamic sharia law.
"Today, we begin preparing for a new phase ... the phase after the liberation, the phase that we will plan and work hard for the future of Libya," said Abdel Hafiz Ghoga, vice chairman of the National Transitional Council.
"Let us start work on the adoption of the constitution," he said late on Sunday as he declared Libya's "liberation" from Gaddafi at a colourful ceremony attended by tens of thousands in the eastern city of Benghazi, where the uprising against the despot was launched eight months ago.
Under the NTC's road map, an interim government is to be formed within a month and elections for a constituent assembly to draft a new basic law held within eight months.
Parliamentary and presidential elections would be held within a year after that – or within 20 months of Sunday's declaration.
NTC leader Mustafa Abdel Jalil insisted that sharia law will prevail in liberated Libya.
"As an Islamic country, we adopted sharia as the principal law," Abdel Jalil told the swarming crowds in Benghazi.
"Any law that violates sharia is null and void legally," he said.
Concerted attempts last month to form a transitional government collapsed due to what Abdel Jalil said were "differences in views" between members of the NTC and the interim executive council.
"We are faced with the Libyan mentality that every tribe, every region, every city has a share in the new government," Abdel Jalil said late September, when the NTC decided to postpone the government-forming exercise until after all of Libya had been freed.
Interim Prime Minister Mahmud Jibril said on Sunday the formation of a new government was expected to take up to a month.
"There are consultations to form a new government and this process would take approximately from one week to one month. It might take longer and or less," he told reporters at the World Economic Forum in Jordan.
"Then there will be real hard work to minimise the period to have elections to elect our national congress, which would be the new parliament instead of the NTC which is going to be dissolved."
US President Barack Obama hailed the liberation as a "new era of promise" and urged a "national reconciliation process," while Secretary of State Hillary Clinton said the revolution was the "work of ordinary, brave Libyans."
Gaddafi's body has been stored in a vegetable market freezer in the eastern city of Misurata, drawing large crowds wanting to view and take pictures of the remains of the despot who ruled Libya with an iron fist.