Another leading critic of Cambodia's government has been charged with defamation.
Pa Nguon Teang was the deputy head of a human rights centre
Pa Nguon Teang, who was arrested on Wednesday, was the deputy director of the Cambodian Centre for Human Rights.
His detention comes just days after police detained his boss, Kem Sokha. Nearly a dozen people have been charged with defamation in recent weeks.
The US has said the arrests call into question the government's commitment to human rights and democracy.
"[Pa Nguon Teang] has been charged with the same crime as his boss," lawyer Som Chandyna told reporters on Thursday.
The defamation charge stems from a banner displayed at a rally to mark International Human Rights Day last month, which allegedly accused Prime Minister Hun Sen of selling Cambodian land to Vietnam.
Yeng Virak, who worked for the Community Legal Education Center, which organised the human rights day, was charged with defamation last week.
Government spokesman Khieu Kanharith dismissed an accusation by Human Rights Watch that Hun Sen had begun to mimic the authorities in the repressive nearby country of Burma.
"We did not ask the court to detain them, that's the court's decision," he told Reuters news agency. "Do you want the government to use the laws, or do you want us to use violence instead?"
Last month, the exiled opposition leader, Sam Rainsy, was sentenced in absentia to 18 months in prison for defaming the leaders of the governing coalition.
This week he appealed to the international community to stop what he called the country's drift into totalitarianism.