Police yesterday arrested Cambodia National Rescue Party leader Kem Sokha over allegations of treason, with Prime Minister Hun Sen accusing the opposition president of conspiring with the United States to oust his regime.

In a startling escalation in an already deteriorating political climate, the premier claimed the government had “no choice” but to detain the CNRP leader, who was arrested just after 12:30am together with eight bodyguards during a raid on his Tuol Kork home. Sokha was taken into custody by more than 100 police and sent almost 200 kilometres away to a prison near the Vietnamese border.

“The treason of colluding with foreigners to betray the nation requires [us] to make an immediate arrest,” the premier told some 4,000 garment industry professionals at an event on Phnom Penh’s Diamond Island.

The prime minister accused the US – whom he often referred to as “the third hand” – of plotting a coup, evoking the US-backed Lon Nol regime, which ousted the late King Norodom Sihanouk in 1970 before being toppled by the Khmer Rouge five years later.

“The third hand used to use Lon Nol to conduct a coup, now the same problem happened,” he said.

“The Americans used to do it, this problem, with Lon Nol and now the American does this problem with Kem Sokha.”

The premier warned the CNRP it faced dissolution if it “dares to appear to protect” Sokha. He also said that other CNRP members, and foreigners, would be investigated for involvement in the alleged plot, saying the matter was “not a joke”.

“We had the option of arresting only one person or sending troops to crackdown at the CNRP headquarters. Arresting only one person is better,” he said. “We call for quiet. If there is any action contrary to the law, the law will be practised.”

Reached yesterday, Ministry of Justice spokesman Chin Malin said Sokha – arrested seemingly without a warrant and in violation of his parliamentary immunity – had been charged under Article 443 of the Criminal Code and could face further offences.

“He was arrested for a red-handed crime,” Malin said, referring to the “in flagrante delicto” clause, which annuls parliamentary immunity.

The clause is traditionally reserved for those caught in the immediate act of committing a crime, but it has been repeatedly used by authorities to arrest opposition politicians despite their protections under the constitution.

“There could be other charges of treason or social insecurity after further investigation,” Malin said.

Article 443 – “conspiracy with foreign power” – is defined as the act of “having [a] secret agreement with a foreign state or its agents, with a view to fomenting hostilities or aggression against the Kingdom of Cambodia”, and carries a 15 to 30 year prison sentence.

Coming 11 months out from national elections, yesterday’s arrest throws into further doubt the legitimacy of the crucial ballot, with the CNRP’s former leader Sam Rainsy already in exile abroad and officially sidelined after controversial legal amendments effectively criminalised his role in the party.

Sokha, who spent almost six months under de facto house arrest last year to avoid jail time in a separate “prostitution” case, now becomes the third opposition lawmaker behind bars. More than two dozen opposition members and critics have also been locked up in the past year.

In a statement yesterday, US State Department spokeswoman Heather Nauert said the charges against Sokha “appear to be politically motivated” and had been noted with “grave concern”.

“This government move follows a number of troubling recent steps, including the imposition of unprecedented restrictions on independent media and civil society,” it read.

“These measures undercut Cambodia’s progress in recent decades and raise serious questions about the government’s ability to organize credible national elections in 2018 which produce an outcome that enjoys democratic legitimacy.”

Though Sokha’s bodyguards were released yesterday afternoon, the CNRP leader was whisked almost 200 kilometres out of the capital to the remote Correctional Centre 3 in Tbong Khmum province’s Trapaing Phlong Prison.

Interior Ministry spokesman Khieu Sopheak said Sokha had been sent to Trapaing Phlong while the probe continued into whether more were involved. “We will investigate the case further,” he said.

The allegations centre on what is framed as a US-backed plot to topple the government, which has been a consistent theme of recent unsubstantiated articles run by government mouthpiece Fresh News that named Sokha, his daughters, several NGOs and journalists as conspirators.

The claims were published first by a Facebook account named “Kon Khmer” – or “Khmer Child” in English – which appeared to have been deleted yesterday.

At the core of the move against Sokha is a video of him giving a speech broadcast by the Australia-based Cambodia Broadcasting Network in late 2013, which reappeared online a few hours prior to his arrest.

Yesterday it was referenced by Hun Sen and in a multitude of statements supporting the arrest by public officials, including top military figures, published by Fresh News.

A version of the CBN clip with English subtitles was released online by the government’s Press and Quick Reaction Unit with the title “Why H.E. Kem Sokha was arrested?”.

In it, Sokha tells an audience in Australia that he had long received assistance from the US and academic experts as part of a political strategy to affect change in Cambodia, but says nothing about toppling the government outside the democratic process.

The then-CNRP vice president said he was encouraged by the US to create the Cambodian Center for Human Rights, which he founded in 2002, and focus his efforts at grassroots change.

“The US say that if you want to change the dictatorial leader, you cannot change the top, you need to uproot to change the bottom first – this is its democratic strategy,” he says in the clip.

In a statement yesterday, CCHR said it adhered to a policy of “strict independence” and expressed concern about Sokha’s arrest, particularly the absence of an arrest warrant.

Several more groups condemned the arrest, including Human Rights Watch, Asean Parliamentarians for Human Rights and the Asian Forum for Human Rights and Development (Forum-Asia).

Slamming the violation of Sokha’s parliamentary immunity, Forum-Asia cited reports from CNRP supporters that the CBN clip had been “heavily edited”.

However, the recently formed “Civil Society Forum” – a group staffed primarily by government officials – called on NGOs to “maintain their neutrality”, while expressing “disappointment” with Sokha’s alleged crimes.

On Twitter yesterday afternoon, Sokha’s daughter Kem Monovithya noted her father had not been permitted visitors and called his arrest “barbaric and illegal”.

In an emailed statement yesterday, former CNRP President Sam Rainsy called the moves a “gross attempt to decapitate the opposition” prior to the elections and called the allegations “baseless”.

CNRP lawmakers, meanwhile, flatly denied the allegations against Sokha, slammed the government for violating their leader’s parliamentary immunity and called for his unconditional release. Spokesman Yim Sovann in an interview with VAYO radio said the party did not want to discuss mass demonstrations and would instead “wait and see” in the hope that the international community could “find a solution”.

However in a separate interview with Voice of America, CNRP Vice President Eng Chhay Eang alluded to the possibility of protests.

“If there is no choice, the demonstration is possible,” said Chhay Eang.

“In context, [Sokha’s speech] has nothing to do with treason. He just discusses and shows about his work and the knowledge that he has received for a peaceful struggle for change. The change is not through the revolution or war but through an election.

“How can it be a red-handed crime since His Excellency Kem Sokha was asleep like a log? This is ridiculous.”

Reporting by Mech Dara, Shaun Turton, Yon Sineat, Phak Seangly, Niem Chheng, Kong Meta, Chhay Channyda, Meas Sokchea and Soth Koemsoeun.