Amnesty International Wednesday urged the "immediate" handover of ousted Libyan leader Muammar Gaddafi's son and his intelligence chief to the International Criminal Court, on the eve of a pre-trial hearing in Tripoli.
"The referral of these cases to the Indictment Chamber brings us one step closer to the start of national trial proceedings... in violation of Libya's legal obligation to surrender him to the ICC," it said.
"It is understandable that the authorities may want to proceed promptly and try these individuals in Libya. But such trials today will not serve justice," Amnesty said.
The London-based organisation said: "Libya's justice system is in desperate need of an overhaul. There are serious concerns about the authorities' ability to ensure fair trials compounded by the precarious security situation in the country.
"Both men should be handed over to the ICC immediately," said Amnesty, referring to Seif al-Islam, Gaddafi's son, and his former intelligence supremo Abdullah al-Senussi.
"Failure to comply with the ICC sends a worrying signal about the Libyan authorities' commitments to protecting and respecting human rights," it said, adding that the group had met both detainees last week.
Libya's public prosecutor has said Seif al-Islam, Senussi and 28 others are to be presented on Thursday to a first instance court in Tripoli, accused of crimes during the 2011 revolt which toppled Gaddafi.
The ICC issued arrest warrants for Seif al-Islam and Senussi in 2011, accusing them of crimes against humanity, and it has a UN Security Council mandate to investigate the Libyan conflict.
But Tripoli and the ICC have been locked in a legal tug-of-war over where the two men should face trial for their roles in trying to put down the bloody revolt against the Gaddafi regime.
The ICC in May rejected Tripoli's request to try the late dictator's son in a Libyan court because of doubts over a fair trial. Tripoli has appealed the decision.
Tripoli's court of first instance, purportedly made up of independent judges, has the power under Libyan law to accept or reject the charges, or to request further investigation.
Seif al-Islam has been held by a brigade of former rebel fighters in Zintan, 180 kilometres (110 miles) southwest of the capital, since his capture in November 2011.