The dock at the Phnom Penh Municipal Court was empty yesterday in the case against opposition lawmaker Um Sam An after the defendant and his team of three lawyers stormed out just minutes into the hearing, decrying the proceedings as “illegal”.

Sam An, of the Cambodia National Rescue Party, was arrested in the early hours of April 11 in Siem Reap, shortly after his return to the Kingdom from abroad, and charged with “incitement to commit a felony (causing chaos in society)” and “incitement to cause discrimination”.

The charges sprang from a number of Facebook posts relating to the controversial Vietnam-Cambodia border, which Sam An, who also holds US citizenship, claimed the government had used incorrect maps to demarcate.

Sam An and his legal team had requested that the judge postpone the case, saying the Supreme Court had yet to rule on whether his parliamentary immunity rendered his initial arrest invalid.

However, judge Heng Sokna chose to continue the trial after prosecutor Kuch Kimlong argued the parliament’s decision to allow authorities to proceed with the case after Sam An’s arrest was binding.

Sam An said he could not cooperate with the court, maintaining that suspension of his immunity required a two-thirds majority vote – as happened last week in the case of opposition Senator Thak Lany.

“This hearing is an illegal hearing, because the trial is against a lawmaker with 100 per cent immunity,” he said.

Before leaving the chamber, defence lawyer Choung Choungy had argued that the arrest, interrogation and detention of his client were unconstitutional.

“I am sure Um Sam An didn’t make any mistake. But the authorities made a mistake in the procedure,” he said. “It is the right of expression. And until now, we have no law about the internet, about Facebook. No law, not guilty.”

But despite the absence of the defendant and his defence team, the trial resumed, with the chamber hearing police testimony and examing the answers Sam An had given police after his arrest. It also looked at his Facebook posts, showed video clips and aired radio interviews concerning the Vietnamese presence on economic land concessions.

The prosecution, unimpeded, argued that the evidence demonstrated Sam An had committed a crime that had caused chaos to the public order. A verdict is due on October 10.

Law expert Sok Sam Oeun said that while conducting proceedings without the defence present smacked of unfairness, “the law cannot favour the accused if the accused walks out”.

“The problem is the accused himself waived his rights . . . he lost an opportunity to defend his claim,” he said.

However, he added, as a pending decision by the Supreme Court could affect the validity of the lower court’s trial, the judge ought to have postponed the hearing.

Government spokesman Phay Siphan said a two-thirds majority vote was not required to strip Sam An’s immunity, as he was arrested for an “in flagrante delicto” crime, and that the Ministry of Interior had waited to arrest him after an extended stay in the United States.

“He happens to be a lawmaker but he’s not mature enough,” he said, maintaining Sam An’s views amounted to incitement against the government that “might jeopardise . . . public order”.

“It is increasing tension between Cambodia and Vietnam,” Siphan said. “You’re not supposed to do that.”