The move sharply escalates political tensions in the Southeast Asian nation and raises questions over whether elections due next year can be free or fair.
The party of Kem Sokha, who police detained during a midnight raid on his Phnom Penh home, denied the allegations and said the charges were politically motivated. 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.
A US State Department spokeswoman, Heather Nauert, did not address the allegations against Washington, but issued a statement expressing "grave concern" over the arrest, saying Ken Sokha "has a long, distinguished, and internationally recognized commitment to human rights and peaceful democracy."
Nauert said the detention and "unprecedented" restrictions recently imposed on independent media and civil society "raise serious questions about the government's ability to organize credible national elections in 2018" Relations between the two countries have deteriorated significantly in recent weeks.
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It has also ordered the English-language Cambodia Daily, founded by an American journalist in 1993, to cease publishing by Monday. It accuses the paper of failing to pay taxes.
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.
Interior Ministry spokesman Gen Khieu Sopheak said Kem Sokha was being held at the Tropeang Phlong prison facility in Tbuong Khmum province, some 130 kilometers (80 miles) east of Phnom Penh. He said the next step will be his court appearance to officially face the charges, which carry a prison sentence of 15 to 30 years.
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