Kem Sokha was arrested in 2017 and his party banned as the government of Prime Minister Hun Sen cracked down on opposition, civil society groups and the media in the run-up to 2018 parliament elections in which the ruling party won every seat.

Reporters were barred from court proceedings after morning sessions on Wednesday and Thursday. A Reuters witness saw some empty seats during the Wednesday session attended by diplomats and officials.

"We are aware that court officials have excluded media and civil society members from the proceedings, and we have urged the court to allow them in," Emily Zeeberg, a spokeswoman for the US embassy told domestic broadcaster Voice of Democracy.

"Freedoms of expression and the press, an active civil society, and tolerance of dissenting views are vital components of any democracy."

The court postponed Kem Sokha's trial proceedings to next week, lawyers for both sides said.

Kem Sokha's treason charges stem from accusations that he was conspiring with the United States to overthrow Hun Sen. He denies the charges, saying they are politically motivated.

As hearings continue, Zeeberg said, the United States urged authorities to address its shortcomings and follow standards of impartiality, transparency, and fair trial guarantees consistent with the constitution.

A coalition of 82 Cambodian civil society groups also urged the court to open the trial to the public, independent media and civil society groups.

In the treason case, government lawyers submitted in evidence a photograph of one of Sokha's daughters having a meal with foreigners, defence lawyers told reporters.

"I eat rice with journalists or with foreigners, and there is a photo, can this be used or not to charge me with treason?" asked Sokha's lawyer Meng Sopheary.

"Eating a meal isn't a crime."

Australian Associated Press