It said evidence of a conspiracy was substantial enough to charge him under the penal code section for “treason and espionage”, which carries up to 30 years in jail.

Kem Sokha was arrested early on Sunday in a swoop by hundreds of security force members at his home in the Cambodian capital.

Hours later Hun Sen – Asia’s longest-serving leader – accused the politician of being in cahoots with the US.

“Behind his [Kem Sokha’s] hand it is still the same, it is America,” he said, in a typically bombastic speech.

To back up his claims, Hun Sen cited a publicly available 2013 speech by the opposition politician given in Australia in which he said he had received US help to build a pro-democracy movement inside Cambodia.

A report on the speech featured on the pro-government Fresh News website soon before the arrest.

Washington has yet to address the espionage allegation but the State Department said the charges against Kem Sokha “appear to be politically motivated”.

It added that his arrest followed “a number of troubling recent steps” against civil society and the free media that threatened to undermine the credibility of next year’s elections.

UN rights chief Zeid Ra’ad Al Hussein said he was “seriously concerned” by the arrest, adding it “appears to have been carried out with no respect for due process guarantees, including respect for his parliamentary immunity”.

Kem Sokha’s surprise detention raises the political temperature in Cambodia.

The CNRP had been widely tipped to perform strongly in the 2018 polls, buoyed by the youth vote in a country where many are tired of rampant corruption, inequality and the dwindling respect for human rights.

In February, the CNRP’s then-leader Sam Rainsy stepped down after a fresh welter of legal cases against him threatened the party with a ban from politics.

He already lives in France to avoid convictions in Cambodia, which he says are politically motivated.

Kem Sokha took over at the helm of the party, but has been buffeted by allegations and threats driven by Hun Sen and backed up by the kingdom’s notoriously pliant courts.

On Monday, a retweet on Kem Sokha’s official Twitter feed read: “I may lose my freedom, but may freedom never die in Cambodia.”

Analysts claim Hun Sen is determined to extend his three-decade rule and withstand the burgeoning popularity of the CNRP, muffling critics in the media and civil society.

“It seems like all the old rules are out the window,” said Sebastian Strangio, an expert on Cambodian politics, adding Hun Sen’s stranglehold on the kingdom is stronger than ever. “Deprived of its leader, and harried by government repression, the CNRP will struggle to mount an effective campaign at next year’s election.”

Hun Sen, a firebrand former Khmer Rouge fighter turned premier, sells himself as the only man who can bind Cambodia together after dragging it from the ashes of civil war and bringing impressive economic growth in the last decade.

That argument has traction among many Cambodians, particularly the older generation who remember the fanatical Khmer Rouge regime of the 1970s which left nearly a quarter of the population dead.