2:30pm: The Cambodia National Rescue Party will place its faith in the international community to “find a solution” following the arrest of its president Kem Sokha, with the party now feeling “powerless”, CNRP lawmakers said this morning.

Speaking to a local radio station, CNRP spokesman and Phnom Penh lawmaker Yim Sovann would not discuss any plans to mobilise supporters to protest Sokha’s imprisonment.

“If the ruling party wants to lead the country down this path we can only watch and see the situation because we have seen the situation from 2014 to 2017, it is not good, we have seen [an] endless series of problems and we are very concerned,” he told Vayo FM radio station.

The decision to “watch and see” rather than call supporters out to the street contrasts with the party’s stance last year when they threatened mass demonstrations after authorities attempted to arrest then-vice president Sokha for ignoring a court summons stemming from a “prostitution” case built on an alleged affair with a hairdresser.

“I do not want to talk about a demonstration and we want to talk to solve things peacefully,” Sovann said. “I want both to be tolerant and to find a solution peacefully because when we do something that causes our people suffering, we should not do it.

“This situation is only the international community can find [a] solution.”

Lawmaker for Battambang Long Botta said the party had no choice.

“Since 1991 Hun Sen has not respected any agreements ... the popularity of the CNRP since the merger in Manila has continued to grow and at each step he tried to block us," he said, referring to the amalgamation of the Sam Rainsy Party with Sokha's Human Rights Party to form the CNRP.

"The red line is not so far away. We are powerless.”

Representatives at the embassies of the US, the EU, Japan, France and Australia have not yet responded to requests for comment.

12pm: Military police are mobilising in Tbong Khmum province, while several military leaders publicly voiced their support for this morning’s arrest of opposition leader Kem Sokha, with one saying soldiers “would not stand still” if “a small group of people” were allowed “to topple the legitimate government”.

The mobilisation follows the decision to send Sokha to CC3 prison in Tbong Khmum’s Trapaing Phlong commune, which a CNRP lawmaker said was likely to avoid a demonstration in Phnom Penh.

Tbong Khmum CNRP activist Chok Huor, 35, said “many” military police and police from the area were stationed along the road to the prison and conducting patrols.

“According to villagers nearby the prison, at night there were many cars travelling back and forth to the prison. They told me Mr Kem Sokha was inside the prison,” Huor said.

“Local activists are very concerned about the arrest of our president Kem Sokha and we are waiting for the orders from the leadership. If there is an order to protest or demonstrate we are ready to participate.”

Meanwhile, in a flurry of statements published by the pro-government outlet Fresh News, several military top brass slammed Sokha, who is yet to be charged but who has been accused of “treason” over what the government says was a plot to conspire with the United States to topple the government.

Under the constitution, the military is ostensibly politically neutral, however almost all senior military commanders are members of the ruling Cambodian People’s Party’s central committee.

Among those to put out similar statements backing the arrest are army commander and deputy supreme commander of RCAF General Meas Sophea, deputy army commander Lieutenant General Prum Din, Military Region 1 commander Lieutenant General Huot Chheang, commander of the Intervention Brigade 1 Lieutenant General Prum Pheng and head of Brigade 70 General Mao Sophan.

Ministry of Defence Department of Military Service head Meas Savorn, head of the military in Prey Vihear Chan Sopheaktra and head of Military Region 5 Bun Seng also released statements carried on Fresh News.

Battambang military commander Uk Khnuoch, quoted by Fresh News, said he “strongly condemned” the opposition leader, citing a 2014 clip of the CNRP president speaking which has re-emerged as part of the government’s efforts to justify the arrest.

In the clip, Sokha talks of receiving support from the United States as part of his efforts to change Cambodia’s leadership.

Khnuoch said the clip “clearly shows [a] secret plan in an attempt to topple the legitimate government with colluding with foreigners”.

“We military commanders, deputy commanders, and all soldiers will not stand still if a small group of people are allowed to continue to do activities that topple the legitimate government,” he said.

“Under the law, we could not tolerate this. We all firmly continue to work to protect the territory and the law of the constitution, and the legitimate government, which is developing the country to achieve total peace.”

11am: Prime Minister Hun Sen has justified the arrest of opposition leader Kem Sokha, saying it was necessary to counter a “national conspiracy” and proved there was the influence of a “third hand” – an oft-used reference to foreign interference – in Cambodia.

The premier was speaking to garment workers at Koh Pich in Phnom Penh, when he said Sokha’s arrest was necessitated by the “seriousness” of the case and that authorities had finally found the “third hand”, although he did not refer to Sokha by name.

“We have been searching for a third hand, who is that? And now we finally found the third hand,” he said.

The arrest followed the circulation of a 2014 video clip on social media, originally published by Cambodian Broadcasting Network in Australia, in which he purportedly admits to getting assistance from the United States to effect a regime change in Cambodia.

“I want you all to broadcast on all the TVs. [The] seriousness is very big and this issue is an act of the national conspiracy,” he said.

The prime minister’s comment seemed to echo his son’s, Hun Manith, who took to Facebook just after the arrest to decry the opposition leader as a traitor.

“Kem Sokha betrayed Cambodia. He confessed to have long term plans with the United States of America… Thank[s] to him, we now know who [is] the Third Hand.”

9:27am: In a dramatic midnight arrest, opposition leader Kem Sokha was taken into custody by around 100 police over accusations of conspiring with a foreign power and treason and sent to Tbong Khmum province’s Trapeang Thlong prison, according to an Interior Ministry official.

Sokha and his bodyguards were taken away after police raided his home just after midnight on Sunday after a 2014 video clip, originally published by Cambodian Broadcasting Network in Australia, was posted to Facebook in which he purportedly admits to getting assistance from the United States to effect a regime change in Cambodia.

While Sokha’s whereabouts were initially unclear, Khieu Sopheak, spokesman for the Interior Ministry, said that a court had ordered the opposition leader be sent to Trapaing Thlong prison in Tbong Khmum province.

“We cannot let him run anywhere, and he had taken money [from] foreigners to organise and follow foreigners to topple the government … He did not use the word ‘vote’, but he said to ‘change’ and ‘topple’.”

A government statement last night said Sokha had been arrested for “conspiracy with [a] foreign power” under Article 443, which comes with a sentence of 15 to 30 years.

“This conspiracy is an act of treason,” the statement read. “[I]t clearly proves the conspiracy between Kem Sokha and the accomplices with a foreign power, which harms the Kingdom of Cambodia.”

The arrest follows weeks of unsubstantiated conspiracy theories put up by Facebook user “Kon Khmer” and parroted by government mouthpiece Fresh News alleging foreign interference through pro-democracy NGOs, and more recently taking aim at Sokha's Cambodia National Rescue Party for being trained by foreign groups to replace the Cambodian People's Party-led government.

The CNRP released a statement condemning the arrest of their leader and called for Sokha’s unconditional release.

CNRP lawmaker Long Botta also confirmed that Sokha had been sent to prison in Tbong Khmum, potentially to prevent any mass demonstrations from being held in Phnom Penh.

He said the speech made in Australia was being portrayed in mostly government-aligned local media outlets as Sokha being a spy attempting a coup to overthrow the government.

“Whether the CPP will eliminate the party, I could not imagine it yet,” he said. “Maybe they try and gather all the testimony or documents so they can arrest all the leaders.”

Human Rights Watch’s John Sifton called the arrest at “setback for Cambodia’s human rights situation”, saying it was time for the international community to put Prime Minister Hun Sen on “notice”.

“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.

Prime Minister Hun Sen’s son, Hun Manith, took to Facebook to decry the opposition leader as a traitor.

“Kem Sokha betrayed Cambodia. He confessed to have long term plans with the United States of America… Thank[s] to him, we now know who [is] the Third Hand.”

Reporting by Mech Dara, Yon Sineat, Phak Seangly, Niem Chheng, Kong Meta, Chhay Channyda, Erin Handley, Shaun Turton, Ananth Baliga.