Updated

Cambodia's opposition leader has been formally charged with treason for allegedly conspiring with the United States to topple the government, and could face up to 30 years in prison if convicted.

Kem Sokha had been expected to lead his Cambodia National Rescue Party in next year's election in a strong challenge against the ruling Cambodian People's Party of Prime Minister Hun Sen, who has held power for three decades.

Kem Sokha's arrest on Sunday came amid a crackdown on the media. Radio stations among the few mass media to carry voices critical of the government were shut down for alleged breaches of regulations, and the English-language Cambodia Daily, also independent of the government, was forced out of business after being presented with a huge but disputed tax bill.