The treason trial of more than 100 members and backers of Cambodia's political opposition commenced Thursday in Phnom Penh.
Groups like Human Rights Watch and Amnesty International condemned the trial as an attempt by long-serving Prime Minister Hun Sen to crush his rivals.
"The dozens of politically motivated arrests demonstrate that Prime Minister Hun Sen's government has no intention of lifting the heavy-handed repression that has darkened Cambodia in recent years," Human Rights Watch's Asia deputy director Phil Robertson said.
"Foreign governments and donors should loudly call for an end to this wave of arrests and press Cambodia to immediately and unconditionally release all those wrongfully detained for criticizing the government."
Rhona Smith, the U.N. Special Rapporteur for Cambodia is worried over the imminent mass trial. She said it was "politically motivated, lacking clear legal grounds and constitutes a serious violation of the due process rights, firmly established by international human rights law."
Smith said mass trials were part of a strategy to intimidate and discredit those who speak out against the government.
"This is not an isolated episode," she said. "Civic and democratic space in Cambodia has continued to shrink and there remains little evidence of political rapprochement and reconciliation," she said on Radio Free Asia.
There are 121 defendants - all associated with the dissolved Cambodia National Rescue Party. They were summoned to appear Thursday but many have fled into exile, convinced they would not get a fair hearing.
Among those who did appear were former opposition senator Thach Setha and Cambodian-American human rights activist and lawyer, Theary Seng.
Mu Sochua, the exiled former deputy leader of Cambodia National Rescue Party told Reuters the trial "will be a showcase with a verdict already decided, not by the judges but by the regime," adding that only 50 defendants would appear.
"More than 120 cases on the same date, the same time, by the same judges cannot be a fair trial," said Sochua, whose trial is at a later date.
Sochua said while she and other exiled leaders had been summoned to court, they are barred from entering the country. "We cannot return to Cambodia without valid Cambodian travel documents. We want to defend ourselves in court. We demand a fair trial," she said.
Security was tight ahead of the trial, with most news media unable to enter what police said was a packed courtroom.
The Cambodia National Rescue Party was banned and its leader Kem Sokha arrested ahead of an election in 2018, ensuring Hun Sen's Cambodian People's Party won every parliamentary seat.
Kem Sokha's treason charges stem from accusations he conspired with the United States to overthrow Hun Sen - claims both he and Washington have rejected.
Other defendants believe they are being punished for advocating the return from exile of Sam Rainsy, Hun Sen's biggest rival during his 35-year rule.
"This is not a legitimate legal proceeding but a show trial, a political theater," Seng, the human rights lawyer, said earlier adding that she had seen no indictment.
"It's a kangaroo court employing laws of the jungle," she told Reuters.
The government says the treason case is legitimate. Justice ministry spokesman, Chin Malin, said the trial would proceed like any other.
The Cambodia government should cease arresting and detaining former opposition party members and rights activists for exercising their basic rights, Human Rights Watch added Thursday.