* Dozens killed at hotel, Human Rights Watch says
* Says events reconstructed from mobile phone videos
* Gaddafi death in crossfire "called into question" -HRW
TRIPOLI, Oct 17 (Reuters) - New evidence implicates Libyan militias in an apparent execution of dozens of detainees in rebel custody following the capture and death of Muammar Gaddafi last year, Human Rights Watch said.
In a report released on Wednesday detailing Gaddafi's final hours on Oct. 20, 2011, the rights group said it had gathered evidence that Misrata-based militias captured and disarmed members of the dictator's convoy and subjected them to brutal beatings.
HRW said militias killed dozens of them in a nearby hotel in what it said constituted the largest documented execution of detainees by anti-Gaddafi forces during last year's conflict.
Gaddafi and his son Mo'tassim were captured in his hometown Sirte two months after rebels seized Tripoli. They died while in the custody of fighters loyal to the country's new leadership, and their bodies were put on display in Misrata.
"The evidence suggests that opposition militias summarily executed at least 66 captured members of Gaddafi's convoy in Sirte," Peter Bouckaert, emergencies director at HRW, said in a statement.
"It also looks as if they took Mo'tassim Gaddafi, who had been wounded, to Misrata and killed him there. Our findings call into question the assertion by Libyan authorities that Muammar Gaddafi was killed in crossfire, and not after his capture."
Libyan officials were not immediately reachable for comment.
Under pressure from Western allies, Libya's former rebel leaders promised to investigate how Gaddafi and his son were killed. But rights groups such as HRW say little has come since.
HRW said the report, "Death of a Dictator: Bloody Vengeance in Sirte", had gathered evidence such as mobile phone footage filmed by rebels, interviews with rebel officers and surviving members of the Gaddafi convoy.
It said one clip filmed by rebel militias showed a large group of captured convoy members in detention being cursed at and abused. It used hospital morgue photos to establish that at least 17 of the detainees visible in the phone video were later killed at the nearby Mahari Hotel in Sirte.
It said a HRW team had visited the site where Gaddafi's convoy of men engaged in a final battle with rebel forces on Oct. 20 and found more than 100 bodies, most killed in combat.
Two days later, the team found the decomposing remains of at least 53 people at the hotel, some with their hands still bound behind their backs. Relatives of some additional dead had recovered their bodies prior to that.
"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.
"Our strongest evidence for these executions comes from the footage filmed by the opposition forces, and the physical evidence at the Mahari Hotel, where the 66 bodies were found."
One case HRW cited was of Ahmed Ali Yusuf al-Ghariyani, a 29-year-old navy recruit from Tawergha, a town Gaddafi forces used to shell Misrata during the war.
"In a phone video that is believed to show him in captivity after the battle, militia forces beat, kick and throw shoes at him, and taunt him about being from Tawergha," HRW said.
"Al-Ghariyani's body was later found at the Mahari hotel, and was photographed by hospital staff and buried as unidentified body number 86. He was later identified by family members from the photographs taken by the hospital staff."
Mobile phone footage also shows Gaddafi and his son Mo'tassim alive after their capture. Gaddafi, whose convoy was hit in an air strike before he was caught in a nearby drainage pipe, was seen being mocked, beaten and abused before he died in what officials said was crossfire.
HRW said footage showed him severely beaten by rebel forces and stabbed with a bayonet in his buttocks; filmed loaded into an ambulance half-naked, it said "he appears lifeless".
It said Mo'tassim was filmed being transported by members of a Misrata militia to their city "where he was again filmed in a room, smoking cigarettes and drinking water while engaged in a hostile conversation with his capturers.
"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," it said.
Libya's new rulers have struggled to impose their authority on a myriad of armed groups who refuse to lay down weapons. Rights groups have accused militias of abusing human rights and cited them as the biggest threat to Libya's democratic path. (Writing by Marie-Louise Gumuchian; Editing by Michael Roddy)