Seen as a move by the regime of Hun Sen — one of the longest serving rulers — to crush opponents, silence critics

Cambodian authorities arrested the leader of the main opposition party on Sunday, accusing him of treason in a move that sharply escalated political tensions in the Southeast Asian nation ahead of national elections next year. His party denied the allegations and said the charges were politically motivated.

The arrest of Kem Sokha during a midnight raid on his Phnom Penh home appeared to be part of a broader push by the government of Prime Minister Hun Sen, one of the world’s longest serving rulers, to crush opponents and silence critics ahead of the vote.

The government in recent weeks has closed down radio stations that gave air-time to the opposition party and threatened to shut down other independent media outlets on charges of violating tax laws that analysts say are politically-motivated.

He was taken hand-cuffed

Mr. Kem Sokha was taken away in handcuffs after a force of between 100 to 200 officers swept his home, according his daughter, Monovithya Kem, who is also a member of his embattled Cambodia National Rescue Party.

Ms. Monovithya Kem said on Twitter that police showed no warrant for her father’s arrest. She said later that his whereabouts were unknown. But Interior Ministry spokesman Gen. Khieu Sopheak said Mr. Kem Sokha was being held at the Tropeang Phlong prison facility in Tbuong Khmum province, some 130 kilometers (80 miles) east of Phnom Penh.

Regime claims incriminating video clip

The government said it had obtained a video clip and other evidence indicating “secret plans of a conspiracy between Kem Sokha ... and foreigners to harm the Kingdom of Cambodia.”

The statement gave no details but called the actions “treason.” Gen. Khieu Sopheak said that Mr. Kem Sokha had admitted in the video that “he was trained and received funding from a powerful foreign country to topple the government.”

“This is a clear crime and there’s no need to make further investigation because he has confessed already,” he added.

Mu Sochua, a vice-president of the opposition party, insisted the charges were fabricated.

She also said that police could not legally arrest Mr. Kem Sokha, who as a lawmaker is entitled to parliamentary immunity. The arrest violated the constitution, she told AP, but “this government has not cared about the law in quite some time.”

‘Cambodia under severe threat’

The arrest “sends a very clear signal that democracy in Cambodia is under severe threat,” she said, adding that if Mr. Kem Sokha it is not released, “it’s very clear there will be no free and fair elections” in 2018.

Analysts say the government of Mr. Hun Sen, an authoritarian leader who held a tight grip on Cambodia for more than three decades, has grown increasingly concerned about steady opposition gains at the ballot box over the last decade, including local elections held in June.

Legal threats forced Mr. Kem Sokha’s predecessor, Sam Rainsy, to resign this year from the Cambodia National Rescue Party he once led. He now lives in exile.

Extremely disturbing development: HRW official

John Sifton, Asia advocacy director at Human Rights Watch, said the arrest was “an extremely disturbing development and a setback for democracy in Cambodia.”

“The government’s charges lack credibility, given its long record of misusing its legal system to silence or intimidate critics and political opponents,” Mr. Sifton said.