Journalists have been turned away from the court where the controversial treason trial of Cambodia’s opposition leader is set to begin.
Reporters from Reuters and the Phnom Penh Post said they had been told there were no more seats available.
Kem Sokha, is accused of plotting to overthrow Hun Sen, who has been in power for 35 years.
Rights group have called the trial politically motivated and a violation of human rights.
Mr Sokha faces up to 30 years in jail if found guilty. The trial itself could last up to three months.
Reuters journalist Chan Thul Prak said he had been told the courtroom was full and was also barred from standing outside the court on this first day of proceedings.
On Monday, court officials said that most of the 30 seats in the court room would be reserved for foreign embassies and consular officials, according to Phnom Penh Post journalist Niem Chheng.
It is still unclear if any journalists from English or Khmer-language outlets have made it into the courtroom.
Why is Kem Sokha on trial?
Mr Sokha and former political rival Sam Rainsy founded the Cambodia National Rescue Party (CNRP) in 2012.
In the 2013 general election, they came within seven seats of victory over the ruling Cambodian People’s Party.
In September 2017, armed police raided Mr Sokha’s home and he was accused of plotting to start a US-backed revolution.
This was based upon an old video where Mr Sokha was seen telling an audience in Australia that he had been receiving political advice from the US.
Hun Sen’s government proceeded to outlaw the opposition, taking all 125 seats in the 2018 election, making Cambodia a de-facto one party state.
What has reaction been to the case?
The case is widely seen as politically motivated.
“Kem Sokha will be the victim of a staged trial on completely bogus treason charges,” said Phil Robertson, deputy Asia director at Human Rights Watch.
“The reality is Kem Sokha did nothing he should have been charged for and this entire pre-trial prison detention, house arrest and now trial has been a massive violation of his human rights,” he added.
Amnesty International said authorities “have not presented a shred of credible evidence to support a charge of treason”.
Cambodia is under international pressure, with the European Union considering the possibility of revoking its preferential trade terms because of Hun Sen’s authoritarian rule.
The outcome of the EU’s review is expected next month.