A closed-door mass trial of 150 Cambodian opposition figures resumed in Phnom Penh Thursday, in what critics labelled a politically motivated sham.

The case is linked to attempts by exiled opposition figurehead Sam Rainsy to return to Cambodia in 2019 from France, where he has lived since 2015 to avoid jail for convictions that he says are bogus.

Many of the defendants stand accused of sharing messages on social media platforms supporting Rainsy's attempt to enter the kingdom.

Prime Minister Hun Sen is one of the world's longest-serving leaders and Thursday marks 36 years since he came to power.

Jailing political opponents and activists has been one method he has used to maintain his position.

Members of the opposition and activists leave a prison van in front of the Phnom Penh court
Members of the opposition and activists leave a prison van in front of the Phnom Penh court AFP / TANG CHHIN Sothy

Many of the accused in Thursday's trial have connections to Rainsy's Cambodia National Rescue Party -- which a court dissolved in November 2017 -- while others are human rights activists.

The charges vary and include "plotting", "incitement to commit a felony", "inciting military personnel to disobedience" and a raft of "criminal attempt" offences under Cambodia's criminal code.

Those convicted of "criminal attempt" face up to 30 years in prison.

Police and security guards removed defendants' supporters from around the courthouse on Thursday, dragging away the tearful wife of a defendant as she tried to catch a glimpse of her husband, held in custody since May.

"My husband is standing trial in the court," the woman, Prum Chantha, shouted at the guards. "I want to meet him, but you guys block me. I feel pain."

Theary Seng, a US-Cambodian human rights activist, faces treason and incitement charges
Theary Seng, a US-Cambodian human rights activist, faces treason and incitement charges AFP / TANG CHHIN Sothy

Theary Seng, a US-Cambodian human rights activist facing treason and incitement charges, was defiant as she walked into court.

"In a society that we call a democracy, I have the right to express my opinions, to express political opinions but I am being persecuted," she told reporters.

"Mr Hun Sen is not the owner of my life. He will not intimidate me with these charges. I will face them and because they are a sham, they are not real charges."

Human rights campaign group Amnesty International said it feared those accused had little hope of a fair trial.

"These mass trials are an affront to international fair trial standards, Cambodia's human rights commitments and the rule of law," a spokesman said.

"This onslaught of cases is the culmination of a relentless campaign of persecution against Cambodia's political opposition and other dissenting voices."

Amnesty said some exiled opposition MPs facing charges had been denied entry into the country to defend themselves.

Further hearings are scheduled for later in January and March.

Meanwhile, Cambodian opposition leader Kem Sokha is facing separate treason charges and has been in custody for more than three years.

His lawyer said the trial had been delayed indefinitely since March last year and authorities could no longer use coronavirus as an excuse.

"The long delay, which is nearly one year now, is an abuse of Kem Sokha's rights and legal process," the lawyer told AFP.