Cambodian opposition leader Kem Sokha, left, is escorted by police at his home in Phnom Penh on Sept. 3 after being accused of treason. (Str/AFP/Getty Images)

The opposition leader is in jail charged with treason, senior party officials have fled abroad and fear is spreading through Cambodia's civil society as a government crackdown intensifies ahead of national elections next year.

The arrest of the president of the Cambodia National Rescue Party (CNRP), Kem Sokha, in a raid of his home just after midnight on Sept. 2 sent shock waves through a country already on edge after attacks on nongovernmental organizations and the media.

In recent weeks, Radio Free Asia and Voice of America have been forced off the air, an independent newspaper, the Cambodia Daily, was shuttered amid government pressure, and the U.S.-funded National Democratic Institute was expelled from the country.

Sokha has been accused of conspiring with the United States to overthrow Cambodia's authoritarian government. He was charged with treason Tuesday and, if convicted, faces up to 30 years in prison while the opposition CNRP could be dissolved under a controversial law passed in February that bans party leaders with criminal convictions.

The 64-year-old is being held in a maximum-security prison near the Vietnamese border, unable to communicate directly with his deputies, several of whom have been named by government-aligned news outlets as being involved with the alleged plot.

His daughter, Kem Monovithya, said the opposition was "no longer functioning" while its leader remains behind bars and a cloud of uncertainty hangs over senior officials.

"The mood in the country, especially among the CNRP officials, is full fear," she said. She is also a party official and has been named on a leaked "blacklist" that is fueling safety concerns.

Referring to the ruling Cambodian People's Party, she said, "They are going to go after the whole leadership until the party cannot function anymore or until the party submits to the CPP, so that we would just be a puppet party that legitimizes the election in 2018."

The opposition plans to boycott an extraordinary session of the National Assembly on Monday that is being held to decide whether the courts can proceed with the case in light of Sokha's parliamentary immunity. Instead, CNRP lawmakers plan to gather outside the prison where he is being held, where government-aligned media report that they will be met by 300 police officers and soldiers.

CNRP Vice President Mu Sochua said the atmosphere was reminiscent of the period leading up to the bloody 1997 coup that saw Prime Minister Hun Sen oust his co-premier, Prince Norodom Ranariddh. Many of the opposition party's most senior officials have left the country, she said, amid the panic. "The prime minister repeats the same thing: We will get to the accomplices of Kem Sokha. He can name anyone he wants. So, who's next?"

The CNRP was formed in a 2012 merger of two opposition parties, and in June it made huge gains in nationwide local elections against the ruling CPP. That came after a surprise strong showing in 2013 national elections, when large numbers of votes cast by young people almost carried the opposition to victory. Months of unrest followed as supporters took to the streets to protest the result, dovetailing with garment sector wage demonstrations that ended when security forces shot into a crowd, killing at least five.

Since then, nongovernmental organizations working on sensitive issues such as labor advocacy and human rights say they have come under increasing pressure. The government has also indicated in recent months that it was investigating such organizations for alleged links to the CNRP.

Naly Pilorge, director of the Cambodian League for the Promotion and Defense of Human Rights, said the government appears to be using almost every tool at its disposal to curtail the work of civil society.

This includes court cases, tax probes, hacking and "aggressive, constant" surveillance. "It's been quite a range and it's intensified at multiple levels," she said, adding that the situation began worsening in the run-up to the June ballot.

Moeun Tola, executive director of the Center for Alliance of Labor and Human Rights, said surveillance had expanded beyond leaders to encompass lower-level employees, who reported being followed and believed their phones were tapped.

"It's not easy to reach out to communities at the moment, as the workers are always blocked or disturbed by authorities," he said.

Ministry of Interior spokesman Khieu Sopheak said that if NGO staff members did not break the law, they would not be under surveillance, dismissing those who made claims to the contrary as "attention seekers."

The government's purported evidence in the case against Sokha is a 2013 video that shows him addressing a group of Cambodians in Australia about his attempts to create political change and the support he had received from the United States over the years.

Sopheak accused the opposition leader's supporters of wanting to mimic popular uprisings of the kind seen in Libya and Syria. "They want to have chaos; they say it is democracy," he said.

Since the CNRP's electoral gains, ruling party officials have been warning of an Arab Spring-style revolt or a "color revolution" that would push Cambodia into civil war. Hun Sen, who has ruled the country for more than 30 years and on Wednesday vowed to continue for another decade, has given regular speeches replete with such warnings.

Lee Morgenbesser, a lecturer at Australia's Griffith University who studies authoritarian regimes, said that "even by the minimum standards" Cambodia could not be described as a democracy.

"The debate, if there is a debate, is to the degree of authoritarianism in Cambodia," he said.

Morgenbesser said the timing of the crackdown was explained by a U.S. retreat from human rights promotion in Southeast Asia while China offers Cambodia large sums of aid and investment with no conditions attached. This, he said, allowed the ruling party to become emboldened at a time when it feels threatened by the possibility of relinquishing power.

In Phnom Penh, the mood on the streets was seemingly unaltered, but concerns bubbled beneath the surface.

"I don't care about politics," said real estate agent Kor Touch, "because when you care about politics, it can cause you trouble."

Phearum Pheang, a cook, said people were aware of the crackdown but did not want to talk about it. "They know that the power is being exercised by only one single person," he said. "You know who they are."

Leng Len contributed to this report.

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