Human Rights Watch (HRW) called on Lebanon Tuesday to release a son of Libya's former dictator Muammar Gaddafi, saying he had been held on "spurious charges" for eight years.

Lebanon in 2015 arrested and accused Hannibal Gaddafi, known for living the high life, of withholding information about the disappearance of Lebanese Shiite cleric imam Mussa Sadr in 1978.

But HRW said he was only two years old at the time the cleric disappeared, and accused Lebanon of subjecting him to an "apparent arbitrary detention on spurious charges".

"Spending eight years in pre-trial detention makes a mockery of Lebanon's already strained judicial system," the group's Hanan Salah said in a statement.

Sadr -- the founder of the Amal movement, now a main ally of Hezbollah -- went missing during an official visit to Libya, along with an aide and a journalist.

Beirut blamed the disappearance on Muammar Gaddafi, who overthrown and killed in a 2011.

Ties between the two countries have been strained ever since.

"It's understandable that people want to know what happened to imam Sadr," Salah said. "But it is unlawful to hold someone in pre-trial detention for many years merely for their possible association with the person responsible for wrongdoing."

A Lebanese judicial official slammed the HRW report as "biased and one-sided", telling AFP it was based solely on "information obtained from Hannibal Gaddafi's defence team".

Hannibal is "detained in a purely judicial matter", the source continued, charging that he was responsible for prisons during his father's rule, "including the one in which the imam was held".

The official said a delegation from the Libyan Ministry of Justice was expected to travel to Beirut in the coming days to meet with the Lebanese justice ministry and the committee overseeing the case to revive a memorandum of understanding from 2014.

In August, Beirut received a letter from Libyan authorities demanding his release, but a judicial source told AFP that he would not be freed before Tripoli revealed information about Sadr's disappearance.

Later that month, Amal movement chief Nabih Berri accused Libya of "failing to cooperate" with the Lebanese judiciary and "concealing" information about the case.