The New York-based watchdog said that during visits by its staff last month, both Seif al-Islam Gaddafi and former spy chief Abdullah Senussi had complained that they had no representation at all during interrogations and pre-trial hearings in their prosecution for gross abuses during the 2011 uprising.
Former premiers Al-Baghdadi al-Mahmudi and Bouzid Dorda said they did have lawyers but that they had been unable to meet them in private to prepare their defence and had been denied access to the evidence against them.
"The prosecution of these men will be no more credible than a kangaroo court if the authorities fail to provide these men with basic due process rights."
HRW called on Libyan authorities to investigate allegations made by Dorda at a court session last month that he had been beaten and wounded in his prison cell.
More From This Section
"Under these circumstances, it's hard to imagine how any of these men can have a fair trial in Libya," Houry said.
No trial date was set at the third and final pre-trial hearing.
Last May, the International Criminal Court rejected a request by Libya to try Seif al-Islam, Gaddafi's former heir apparent, on war crimes charges because of doubts he would receive a fair trial.
Tripoli has appealed the decision but Seif is still wanted for trial by the ICC on charges of war crimes and crimes against humanity during the 2011 uprising that ended his father's 42-year rule.