Sam Rainsy wrote on Facebook on Thursday that he thought the case was politically motivated and carried out by a “puppet court”.
Under Cambodian law, Nguyen Thanh Dung could be charged with aggravated torture and acts of cruelty, which carries a sentence of 10 to 20 years imprisonment.
Palm oil's rapid growth has spawned huge plantations, especially in Southeast Asia. Amnesty International's Meghna Abraham says the burgeoning demand also is driving widespread labor abuses.
Presiding Judge Kim Sathavy said the decision was made to prevent the accused from interfering with the investigation.
Normally, prisoners must be released after a maximum of six months if the courts have failed to gather enough evidence to take the case to trial.
Hang Puthea, National Election Committee (NEC) spokesman, said there had been a lack of cooperation between the authorities and campaigners.
Thak Lany, a senator for the Sam Rainsy Party, was sued by Hun Sen in early August after a video emerged, posted on pro-ruling party website Freshnews, that purported to show Lany blaming Hun Sen for the July 10 killing.
A court in Phnom Penh on Thursday held a hearing in the case against Tep Vanny, a prominent housing activist from the Boeung Kak Lake area. The court denied her request for bail.
Hang Puthea, NEC spokesman, said those who missed out of registering ahead of the 2017 elections would still be able to register for the 2018 general election.
The defendants argued that the plaintiffs had failed to present a case for extraterritorial jurisdiction.
The Court of Appeals denied the bail request, which was made on a number of grounds, including affects on the defendants’ health, after a brief hearing.