The trial of Cambodia's Opposition Leader on charges of treason is underway, drawing condemnation from rights groups who say the case is a move by longtime leader Hun Sen to crush his political rivals.

Key points: Kem Sokha is accused of plotting with the United States to overthrow Hun Sen

Kem Sokha is accused of plotting with the United States to overthrow Hun Sen Critics say Hun Sen, who has led Cambodia for 35 years, is turning it into a one-party state

Critics say Hun Sen, who has led Cambodia for 35 years, is turning it into a one-party state The EU will soon decide whether to cut trade with Cambodia over the political situation

Diplomats packed the Phnom Penh Municipal Court as the defendant, Kem Sokha, arrived, with only a few journalists allowed in to view the proceedings.

It could take up to three months to deliver a verdict, defence lawyers said, meaning the case could still be going by the time the European Union makes a decision next month on whether to cut Cambodia's preferential trade status over its human rights record.

Mr Sokha was arrested in 2017 and his Cambodia National Rescue Party was banned ahead of a 2018 election in which the Prime Minister's party won all the parliamentary seats.

He was freed from house arrest in November but a ban on him engaging in political activity was kept in place.

Only a handful of journalists were allowed inside court to report on the proceedings. ( AP: Heng Sinith )

The treason charges stem from accusations Mr Sokha was conspiring with the United States to overthrow the Prime Minister. He denies the charges, saying they are politically motivated.

Rights groups have decried the case. Phil Robertson, deputy Asia director of New York-based Human Rights Watch, said it was unlikely Mr Sokha would receive a fair trial.

"Kem Sokha will be the victim of a staged trial on completely bogus treason charges," Mr Robertson said.

"The Government should cut their losses by dropping the charges against him, but clearly Prime Minister Hun Sen decided instead to extract the proverbial pound of flesh from Kem Sokha for daring to challenge him."

Hun Sen, who has ruled Cambodia for 35 years, has been accused of setting up a one-party state. ( AP: Heng Sinith )

Ministry of Justice spokesperson Chin Malin declined to comment on the trial when asked on Tuesday.

Many other opposition figures have gone into exile and have accused the Prime Minister, who has ruled Cambodia for 35 years, of establishing a one-party state.

The European Union, which accounts for nearly half of Cambodia's exports, will decide in February whether or not to remove Cambodia from its Everything But Arms trade scheme over its crackdown on political expression.

The only evidence presented against Mr Sokha in public so far is a comment he made at a public meeting in which he said he was getting advice from unspecified Americans on election strategy ahead of a 2013 ballot.

Mr Sokha's daughter Monovithya Kem called the trial against her father a "farce" as rights groups called for the case to be dismissed.

Reuters