PHNOM PENH (Reuters) – Cambodian opposition leader Kem Sokha was arrested in a police raid on his home early on Sunday and veteran Prime Minister Hun Sen accused him of treason with the backing of the United States.

The arrest marks an escalation in a campaign against critics, independent media and any potential threats to Hun Sen’s hold on power ahead of an election next year at which Kem Sokha has been expected to be his main challenger.

“It’s an act of treason with conspiracy with a foreign country, betraying his own nation. This requires arrest,” Hun Sen told a group of garment workers according to the pro-government Fresh News website.

Hun Sen said of the alleged foreign third party: “It’s the United States.”

Hun Sen, 65, has ruled the Southeast Asian country for more than three decades. The former Khmer Rouge cadre has become one of China’s closest regional allies and has been making increasingly strident verbal attacks on the United States.

Kem Sokha, 64, has led the main opposition Cambodia National Rescue Party (CNRP) since his predecessor resigned in February, saying he feared a government plan to shut it down.

Pictures in Cambodian media showed Sokha being led away with his hands behind his back.

The government said in a statement that it had a video clip and other evidence pointing to a secret plan with foreigners to harm the Kingdom of Cambodia which amounted to treason. Details of the alleged plot have not been given.

The opposition party said Kem Sokha’s arrest was politically motivated and violated the law because his position as an elected lawmaker gave him immunity from prosecution.

The party called for Kem Sokha’s immediate release and urged the international community to “intervene for the release of Kem Sokha” and to stop official intimidation of party leaders.

It said it was committed to a non-violent approach.

Kem Sokha’s daughter, Monovithya Kem, who is also an official in the party, said on Twitter that her father had been taken away handcuffed after a raid by between 100 and 200 police, who had arrived without an arrest warrant. She said his whereabouts was unknown.

Kem Sokha made no immediate comment and it was not clear if he had legal representation at this stage.

U.S. ACCUSED

Fresh News said it had video of Kem Sokha discussing overthrowing Hun Sen with support from the United States.

Neither the U.S. State Department nor the White House responded immediately to a request for comment.

The government has recently increased its rhetoric against the United States and last month ordered the expulsion of the U.S. State Department-funded National Democratic Institute pro-democracy group. Earlier in the year, it suspended joint military exercises with the United States, which has voiced fears over the human rights situation.

“Freedom of speech is rapidly becoming a highly endangered right in Prime Minister Hun Sen’s march down the road to dictatorship in Cambodia,” said Phil Robertson, Deputy Asia Director of the Human Rights Watch campaign group.

Last month, Hun Sen’s government stepped up attacks on the media, halting broadcasts by some radio stations and ordering an independent newspaper, The Cambodia Daily, to close if it did not pay a $6 million tax bill within days.

During Hun Sen’s rule Cambodia emerged from the devastating Khmer Rouge genocide to enjoy record years of economic growth of above 7 percent, but disaffection has been growing and he only just won the 2013 election against a unified opposition.

His Cambodian People’s Party also won local elections in June, but the opposition also did well, increasing expectations of a close contest in the general election due in 2018.

Kem Sokha took over the party leadership after his predecessor, Sam Rainsy, resigned in February. Sam Rainsy said he had quit to save the party in the face of a threatened ban on any party with a leader who has been convicted of a crime.

Sam Rainsy lives in exile in France to avoid a defamation conviction he says was politically motivated.