PHNOM PENH -- Cambodian authorities are planning to "strictly control" the activities of potential critics of the regime, as Prime Minister Hun Sen aims to further strengthen the power base of his Cambodian People's Party, according to a leaked government document.

Meanwhile, leading members of civil society groups this week told the Nikkei Asian Review about widespread surveillance and intimidation tactics being used against people the authorities see as political critics.

The internal memo, written by Phnom Penh's police chief Lt. Gen. Sar Thet, was leaked late last month and outlined plans for more than 500 officers to "safeguard" Kem Sokha, leader of the outlawed opposition Cambodia National Rescue Party.

Kem Sokha was recently put under house arrest after more than a year in pretrial detention on widely discredited claims that he was plotting a U.S.-backed coup.

While mainly detailing measures to prevent "all tricks and action plans concerning terrorism, crime and causing social chaos" in relation to Kem Sokha's release, the document also mentions broader plans to curtail potential dissent in society.

The authorities were to "strictly control activities of civil society organizations, unions, foreigners, former CNRP leaders and the 118 individuals involved in the case who were banned from politics," it read.

The document comes to light after the CPP won all 125 National Assembly seats in the July 29 general election. The vote was widely criticized as flawed by Western governments, in large part due to the absence of the CNRP, which had come close to a shock victory in the 2013 election.

The ballot took place following a year of intensified repression of political opposition, government critics and independent media that saw dozens jailed or fleeing the country.

There has since been an escalation in the surveillance of human rights groups and critics of the regime, sources said.

"We always have spies observing our activities and also my colleagues in the provinces," said Soeng Sen Karuna, a senior official of rights group Adhoc. "They threaten us and they are observing what we are doing."

Sometimes the "spies" stand outside his office disguised as tuk-tuk or motorcycle taxi drivers, he claimed.

"Sometimes, when staff leave work, [the spies] try to follow. For me, I look in the mirror when I ride my motorbike and they follow, I see, but then I escape. I do not ride the same way, sometimes I go a different way," he said.

Meas Nee, a political analyst and researcher, said he had heard widespread complaints of local authorities disrupting or shutting down meetings involving rights groups. In the past six months, some local officials had started demanding such meetings take place in the house of the head of the commune, the local administrative division.

"The commune chief is sleeping upstairs so he can listen to what you're talking about. It is really strict," he said.

Nee said he and his peers are now far more wary of meeting in public places due to fears of government agents listening.

"Once the meeting has finished, each [agent] has to follow one of us to see where you go from here. They did it before the election, they did it after the election, they did it after [political analyst] Kem Ley was killed, they did it at the time they arrested Kem Sokha -- you could see the presence of secret service staff," he said.

A source, who requested anonymity for fear of reprisals, said they had heard from sources in Cambodia's intelligence network that a meeting had been penciled in for the end of this month to draw up a list of individuals to be monitored.

Chak Sopheap, executive director of the Cambodian Center for Human Rights, said that the authorities were mixing "traditional" techniques such as physical surveillance and intimidation with "modern" approaches including the use of digital tools and the judicial system to silence dissent.

Any hopes that space for critical voices would re-emerge after the election had been quashed, she added.

"The constant monitoring, intimidation and harassment of [civil society organizations], and the corresponding lack of transparency and accountability when it comes to attacks against CSOs, create an atmosphere of fear and insecurity," she said.

National Police spokesman Lt. Gen. Chhay Kim Khoeun told The Phnom Penh Post last month that the capital's municipal police were responsible for the plan and that he "had nothing to explain."

Government spokesman Phay Siphan said that only suspected terrorists were under surveillance, adding that the concerns of rights groups were misplaced.

"They feel afraid themselves; it's just an allegation to accuse the government. NGOs are welcome here," he said.

However, such groups were required to keep the authorities up to date with their movements, he added.

"Wherever they go, they just need to inform," he said. "They do not need to go by themselves. We mind that much about our national security."