A Cambodian opposition leader, Kem Sokha, has denied conspiring with foreign powers, as his treason trial began in Phnom Penh.
"I did NOT bring in foreign states or foreign agents to conduct hostility or invasion against Cambodia," said the statement on Facebook.
Most reporters were barred from court, but his lawyer revealed Mr Sokha was accused of receiving US support.
Rights groups say have described the trial as a violation of human rights.
The accusation that Mr Sokha received long-term support from the US was based on a video of a speech he gave in Australia in 2013, his lawyer told journalists outside court during a break in proceedings.
The courtroom was shown a short clip of the video provided by the prosecutor, local media reported.
Mr Sokha's lawyers have asked for the full, hour-long version to be shown.
The opposition leader is accused of plotting to overthrow Cambodian leader Hun Sen, who has been in power for 35 years.
Journalists said only a "few" of their colleagues were allowed into the court. The trial itself could last up to three months.
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.
What did Mr Sokha say?
In the Facebook statement, Mr Sokha "categorically denied" the charge of "conspiracy with foreign powers".
"I did NOT commit anything of which I am accused - especially I haven't committed any acts that would be detrimental to the national interests," he said.
He added that all his activities were "focused on human rights and democracy, carried out in peaceful and non-violent manners".
What is he accused of?
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 on the 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 him 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.
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."
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.
Why weren't journalists allowed in?
All observers were required by the court to register in advance.
Court officials said most of the 30 seats would be reserved for foreign embassies, 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.
I won’t be allowed to cover Kem Sokha’s trial proceedings, courtroom is full, police said as journalistsare chased away from even standing in front of the court pic.twitter.com/ps9CkFi9XP— Chan Thul Prak (@prakchanthul) January 14, 2020