A mass trial of more than 120 Cambodian opposition figures kicked off in Phnom Penh Thursday, amid UN concerns the proceedings were politically motivated and violated due process.

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

Prime Minister Hun Sen is one of the world's longest-serving leaders, maintaining a 35-year grip on power with methods that include jailing political opponents and activists.

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.

Many defendants have fled the country fearing arrest.

Ney Leak, 29, an opposition political activist who faces charges over posting messages supportive of Rainsy's return, said she was "innocent".

"These are serious charges. I hope the court will drop it because we have done nothing wrong," she told reporters.

American-Cambodian human rights activist Theary Seng is accused of conspiracy to commit treason and incitement to commit a felony.

"I have done nothing wrong... it's a sham trial. It's political theatre. It is a political circus," she said.

United Nations Special Rapporteur on the situation of human rights in Cambodia Rhona Smith said since June 2019 more than 150 people associated with the dissolved opposition party have faced arrest.

"The mass trials of activists appear to be politically motivated, lacking clear legal grounds and constitute a serious violation of the due process rights, firmly established by international human rights law," she said, adding the proceedings were part of a strategy to intimidate and discredit opponents of the government. 

After two hours the judge split the case in two, following lawyers' requests for more time to prepare.

Some defendants sought permission to choose their own legal representation rather than use ones appointed by the court.

Hearings are expected to resume in January and March.