Global Witness alleges that Hun Sen has used his position as prime minister to enrich his family's wealth.
Their arrest sparked a protest movement known as the Black Monday campaign.
Research by the Cambodian Center for Human Rights suggests Black Monday activists were arrested 33 times last year.
Cambodian People’s Party Senator Ly Yong Phat told VOA Khmer that he was not connected to the dispute and that the demonstration was “political”.
Activists and workers say they will never forget the mistreatment at the hands of the security forces, such as the Special Forces Division 911 on January 2.
No serious injuries were reported when the security forces moved the protesters from the building Tuesday.
The killing of Kem Ley has come to symbolize the ills of Cambodian society under Prime Minister Hun Sen’s 31-year rule.
In 2014, Sahmakum Teang Tnaut (STT)estimate that almost 30,000 people had faced forced eviction in Phnom Penh, since 1990.
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.