PHNOM PENH, Cambodia — The leader of Cambodia’s main opposition party, Kem Sokha, was arrested early Sunday at his home and accused of treason for participating in what the government said was a Washington-backed plot to undermine the country’s leadership.
A top official for Human Rights Watch in Asia called the arrest a “disastrous setback” for Cambodia as its long-ruling authoritarian prime minister, Hun Sen, consolidates power in advance of elections scheduled for next year and marked a further deterioration in its relations with the United States.
Mr. Kem Sokha took over as the leader of the Cambodia National Rescue Party earlier this year, after his longtime predecessor, Sam Rainsy, who is living in exile, resigned from the post after being pursued in several criminal cases.
Mr. Kem Sokha’s daughter, Kem Monovithya, who also is a party official, said more than 100 police officers arrested her father and his bodyguards without a warrant. A video posted on Facebook showed Mr. Kem Sokha wearing handcuffs as he was escorted into an unmarked sports utility vehicle by officers wielding AK-47 assault rifles.
“Kem Sokha and all bodyguards are taken away by 100-200 police without warrant after they raided his home,” she posted on Twitter shortly after his arrest. “We don’t know where they take him.”
Late Sunday, she said, her father had been taken to a remote prison on the Vietnamese border known as CC3 and was being held without access to a lawyer.
The government said it had evidence that Mr. Kem Sokha was involved in a secret plot and had conspired with foreigners to damage the country. “The above act of this secret conspiracy is treason,” the government said in a statement.
Although Mr. Kem Sokha has parliamentary immunity, the government said he could be arrested without a warrant because he was caught committing treason “in flagrante delicto.” As evidence, it cited a four-year-old video of Mr. Kem Sokha, in which he says in a speech that he received advice from the United States on building an opposition movement.
Arend Zwartjes, a spokesman for the United States Embassy in Phnom Penh, had no comment about the accusations by Cambodia that Washington had conspired with Mr. Kem Sokha. But he said that the government had noted the arrest “with grave concern,” adding that the charges “appear to be politically motivated.”
Founded in 2012, the Cambodia National Rescue Party poses the first genuine challenge in years to Mr. Hun Sen’s rule. In an interview with The New York Times in June, Mr. Kem Sokha said he hoped his party could win parliamentary elections next year and effect a peaceful transfer of power.
“The bottom line is that the people have already decided for change,” he said. “That is, they have already made up their mind, and if the ruling party is in some way preventing that change through free and fair elections, the international community needs to use all the leverage they have to counter that in order to avoid violence.”
Speaking to a group of garment workers on Sunday, Mr. Hun Sen reiterated the accusation that Mr. Kem Sokha had conspired with the United States. A similar claim was made on the official Facebook page of the prime minister’s son Hun Manith, a two-star general and head of military intelligence for the Cambodian Army. “Kem Sokha betrayed Cambodia,” he wrote, adding that he confessed “to long term plans with the United States of America.”
David Chandler, a professor emeritus at Monash University in Australia who worked as a United States Embassy official in Phnom Penh in the 1960s, said that the situation in Cambodia had become “genuinely scary” in recent months.
He said that Mr. Hun Sen has long been suspicious of Washington’s intentions and believes that a strong element of United States policy is regime change.
“I can’t see why or how Hun Sen will turn back from where he seems to be headed,” Mr. Chandler said, “and I don’t think he wants to.”
In recent weeks, Mr. Hun Sen’s government has cracked down on independent news outlets and democracy advocates, after local elections in June in which the opposition party made unprecedented gains.
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An independent newspaper owned by an American family, The Cambodia Daily, said it would halt publication on Monday because of a dispute over a tax bill of more than $6 million, and at least 15 radio stations have been ordered to stop broadcasting programming from the Voice of America and Radio Free Asia.
The government also ordered the expulsion of the National Democratic Institute, a pro-democracy nonprofit organization that is loosely affiliated with the Democratic Party of the United States.
It has become common in recent months for Cambodian officials to accuse the United States of hypocrisy and interference in Cambodian affairs. A government spokesman, Phay Siphan, denied any tensions with the United States, however, describing the recent developments as “a coincidence” and proof of the government’s commitment to rule of law.
Mr. Kem Sokha’s arrest was quickly condemned by opposition leaders and international human rights activists.
“The arrest of Kem Sokha is a disastrous setback for Cambodia’s human rights situation,” said John Sifton, Asia advocacy director at Human Rights Watch.
He urged other nations to put pressure on Mr. Hun Sen. “The international community, which provides a major percentage of the Cambodian government’s annual budget, should put Hun Sen on notice that if he doesn’t reverse course, it will be impossible to consider next year’s elections free and fair,” he said.
The arrest also drew fire from Asean Parliamentarians for Human Rights, an association of members of Parliament from Southeast Asian nations.
Charles Santiago, a member of the Malaysian Parliament and chairman of the organization, said the arrest violated the immunity protection that should be afforded Mr. Kem Sokha as a member of Cambodia’s Parliament.
“With national elections on the horizon, it is clear that this is nothing more than a thinly veiled attempt to crush the opposition before the campaign even starts,” Mr. Santiago said. “For months, we have been witnessing the escalation of government attempts to cripple the opposition, but it appears now that Prime Minister Hun Sen is so afraid of what might happen in a genuine vote, he won’t allow for competition at all.”
Neou Vannarin contributed reporting.