Kem Sokha, who is the vice president of the Cambodia National Rescue Party, says the allegations have been trumped up by the ruling party in order to discredit the opposition ahead of July 28 national elections.
Sou Met, who commanded the Khmer Rouge air force, was accused of major atrocity crimes as a ranking member of the regime.
Court observers say they are skeptical of a court that has seen routine delays and completed just one trial since 2006.
Phnom Penh Municipal Court has opened a hearing against senior opposition official Kem Sokha, who is being sued by a Khmer Rouge survivor for allegedly denying atrocities committed by the regime.
The request was made to Prime Minister Hun Sen by lawyers for civil party participants at the tribunal.
In an interview with VOA Khmer, Poch Yuon Ly’s daughter, Poch Piseth Neary, called the diary, “the only valuable asset my father left me.”
In recent weeks, Kem Sokha has been accused of claims that atrocities at the prison were staged by Vietnamese forces after they ousted the Khmer Rouge from power.
Hun Sen said in a public speech Thursday the demonstrations calling for an apology for his alleged remarks should be postponed until after the election.
Pressure continues for a senior opposition party official to apologize for alleged remarks that has upset victims of the Khmer Rouge.
Thousands of government supporters protest in Cambodia against comments allegedly made by opposition leader about atrocities during the Killing Fields era. VOA Khmer's Heng Reaksmey reports from Phnom Penh.
The law was passed by all 86 lawmakers in attendance, from the ruling Cambodian People’s Party and the royalist Funcinpec.
More than 12,000 people were tortured and sent to their executions at the center overseen by Duch, who was found guilty for his crimes by the UN-backed tribunal in February 2012.