Opposition Senator Hong Sok Hour was sent to Prey Sar prison yesterday and charged with forgery and incitement, just three days after Prime Minister Hun Sen publicly accused him of treason.

In a report filed to acting Senate President Nay Pena, Phnom Penh Municipal Court prosecutor Yet Chakrya said the Sam Rainsy Party senator had been charged under three articles of the Kingdom’s Criminal Code, which could carry a combined maximum prison sentence of 17 years.

“After examining the evidence collected, we can see that the above case involved crimes of forging a public document, using a forged public document, and incitement to cause serious unrest for social security,” Chakrya wrote.

Sok Hour went into hiding after Hun Sen, in a speech on Thursday morning, accused him of posting a “fake” section of the 1979 Cambodia-Vietnam border treaty to his Facebook page on Wednesday, and called for his “urgent” arrest.

The senator was arrested at the home of Cambodia National Rescue Party (CNRP) lawmaker Yon Tharo early on Saturday morning by armed police.

In a separate letter to Pena yesterday, Phnom Penh Municipal Court investigating judge Kor Vandy confirmed the three charges and Sok Hour’s subsequent detention in Prey Sar’s Correctional Centre 1, which he said was decided on in accordance with the law.

However, defence lawyer Choung Choungy yesterday said the action flew in the face of protections awarded to Senators under the constitution.

“It is not legal,” he said. “His detention is contrary to article 104 of the constitution, which states that senators have immunity [and] cannot be detained.”

Article 104 grants senators broad-based immunity from arrest, except in cases when the body itself approves it, or if a senator is caught red-handed in a criminal act.

Choungy said he will appeal against the court’s decision today, and attempt to secure bail by paying an undisclosed fee.

However, despite Senate immunity, observers yesterday said they were unsurprised by the action against Sok Hour.

“Basically, all of these charges follow the lead of what was said by Hun Sen,” said independent analyst Ou Virak.

Virak said Sok Hour had showed “really poor judgment” by posting the documents “with no basic check of whether they were real or not”, but argued that “he should be told to apologise” rather than be sent to prison.

He added that the measures against Sok Hour could be interpreted as a message from the premier that further action over the hotly contested Vietnamese border areas will not be tolerated.

“Maybe this is Hun Sen closing that chapter,” he said.

As Sok Hour was sent to pre-trial detention yesterday, CNRP president Sam Rainsy returned from a trip to France, vowing to attempt to secure the release of the senator and 14 other CNRP members currently imprisoned on “insurrection” charges.

“I came to initiate ways to make all the detainees be released.

The result I want to see is for the detainees to be released all together,” he said.

The opposition leader said he plans to meet with Hun Sen to resolve the issue, but did not reveal details of when that might be.

Despite the ongoing arrests of his members, Rainsy once again defended the so-called “culture of dialogue” between the two parties – a period of uneasy detente that has seen the opposition temper its criticism of the ruling party in recent months – arguing that it could be used to secure their release.

“Twice in the past, in July 2014 and April 2015, through the culture of dialogue, we reached a solution to the issue resulting from the arrest and detention of a number of CNRP officials and activists,” he wrote on his Facebook page.

“I hope that this time, in August 2015, a similar solution to a similar problem could also be reached.”

ADDITIONAL REPORTING BY ALICE CUDDY