Bangkok:  Cambodian strongman Hun Sen has widened a crackdown on his critics, forcing half of the country's opposition politicians to flee overseas in fear of arrest.

The latest to join the exodus is Mu Sochua, a respected and outspoken deputy leader of the Cambodian National Rescue Party (CNRP), which has been left decimated ahead of general elections next year.

Mu Sochua, 63, said after flying out of Cambodia that "without a true opposition and with the wide level of fear, there's no hope for free and fair elections in 2018".

"Democracy in Cambodia is very rapidly eroding to a point where no other opposing forces are left to fight dictatorship," she said, adding she had been told her arrest was imminent.

Mr Hun Sen, a former commander of the murderous Khmer Rouge who has ruled the country with an iron-fist for 32 years, has vowed to arrest alleged "rebels" and "foreign slaves" who he claims are part of a US-backed conspiracy to overthrow him.

But human rights groups say Mr Hun Sen has embarked on a campaign of repression that is sliding the south-east Asian country into a dictatorship.

The Turnbull government is this year sending $89 million in development aid to the regime in Phnom Penh, one of the world's most corrupt. 

Canberra has paid the regime a further $55 million as part controversial deal to send refugees to resettle in Cambodia, but only a handful have agreed to make the journey.

Ms Sochua, known for campaigns to fight sex trafficking and assert women's rights, told Fairfax Media in September that millions of Cambodians were concerned about the future of their democracy.

"We appeal to Australia and other countries to help us," she said.

Ms Sochua had made several political speeches in Cambodia's provinces over the past week.

Her decision to leave came after Kem Sokha, the leader of her party, was arrested on treason charges in an early morning commando-style raid on his house by 200 armed police on September 3.

Authorities at the time released a video of a speech Mr Sokha made in Australia in 2013 that allegedly showed him talking about plans to topple the government

In his first comments from jail, Mr Sokha said the raid on his house was a violation of his right to privacy and his parliamentary immunity.

 "On the point that the authorities charged me, of colluding or conspiring and working with foreigners to topple the government, it is completely slanderous," he said.

Mr Hun Sen, 65, warned in a speech on Monday that more arrests were imminent over an alleged plot to overthrown him.

"This isn't over with one arrest. I would like to send out a message," he said.

"This set-up is systematic activity."

Mr Hun Sen also warned Cambodians "do not do whatever you want to do and serve as a foreign slave to destroy Cambodians".

The US embassy in Cambodia has rejected any suggestions the US is interfering in the country's politics.

Mr Hun Sen has developed close ties to China which has sent hundreds of millions of dollars in aid and investment to the country of 16 million mostly impoverished people.