Cambodian Prime Minister Hun Sen vowed on Monday to resist foreign interference, following worldwide criticism of the arrest of his main rival for treason and a widening crackdown on his critics.
According to a government statement, Sokha was accused of "treason" based on a years-old video clip circulated online in which Sokha said he was encouraged to enter politics during a meeting with US representatives.
Cambodian authorities arrested the leader of the main opposition party on Sunday, accusing him of conspiring with the United States to topple the government, on the same day that the government forced The Cambodia Daily, an independent newspaper, to shut down.
Analysts say Hun Sen, an authoritarian leader who has 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 in June.
Kem Sokha's daughter, Monovithya Kem, who is also a member of his embattled Cambodia National Rescue Party, said her father was taken away in handcuffs after a force of between 100 to 200 officers swept his home in a surprise raid.
"We don't know where they take him", she said.
Mr. Kem Sokha's arrest was quickly condemned by opposition leaders and global human rights activists.
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In its statement on Sunday, the CNRP vehemently condemned Sokha's arrest, saying that it "is politically motivated and violates law and constitution" because Sokha is a lawmaker who has parliamentary immunity. The government also ordered the expulsion of the National Democratic Institute, a pro-democracy, nonprofit organization tied to the Democratic Party of the United States. Earlier in the year, it suspended joint military exercises with the United States, which has voiced fears over the human rights situation.
He urged other nations to put pressure on Mr. Hun Sen.
Nauert also highlighted recent steps by the Cambodian government against independent media and civil society, saying they "raise serious questions about the government's ability to organize credible national elections in 2018 which produce an outcome that enjoys democratic legitimacy".
The arrest 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 eliminate his opponents and silence critics ahead of the 2018 vote.
In the run up to crucial general elections next year, strongman Hun Sen has sought to stifle opposition voices.
The English-language paper had been given a deadline of one month to pay $6.3 million for years of back taxes, which the publication disputed and described as "astronomical".
Journalists work at the newsroom of The Cambodia Daily newspaper in Phnom Penh, Cambodia, September 3, 2017.
In August, at least 15 radio stations broadcasting content from Radio Free Asia, Voice of America, Voice of Democracy, and other foreign networks were shut down by the government for "violation of contracts" with the information ministry.