Some journalists have been turned away from the court where the controversial treason trial of Cambodia’s opposition leader has begun.
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.
Journalists said only a “few” of their colleagues were allowed into the court, with others waiting on the street outside.
“The authorities are clearly trying to limit scrutiny of this politically-motivated trial,” said Nicholas Bequelin from Amnesty International.
“It is essential that human rights monitors and journalists are given unhindered access to the trial.”
Mr Sokha faces up to 30 years in jail if found guilty – although some analysts say he could be convicted and pardoned, in order to protect a trade agreement with the EU.
The trial itself could last up to three months.
What have journalists said?
All observers were required by the court to register in advance.
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, who said he was not allowed to enter.
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.
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 received 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 and the daughter of Kem Sokha has called the trial against her father a “farce”.
“We hope that he will be acquitted if the court has any interest at all in starting to demonstrate its independence,” said Monovithya Kem according to Reuters.
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Phil Robertson, deputy Asia director at Human Rights Watch said: “Kem Sokha will be the victim of a staged trial on completely bogus treason charges.”
“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.