Dead dissidents dumped in a river, activists knotted up by the courts, and Big Brother-style internet laws -- critics of Thailand's junta fear this week's election is poised to sharpen the dangers faced by those who disagree.

Thais goes to the polls on March 24, in the first election since the 2014 coup that installed the generals in power.

But it will be held under new rules established by a junta that has made clear it has no intention of leaving the political stage.

Scores of dissidents, academics and "Red Shirt" activists have been pushed into self-exile during the junta years, in what analysts say is one of the biggest political flights in Thailand's recent history.

Some found sanctuary in the West, but the majority fled to neighbouring countries to avoid charges and jail terms.

"I couldn't bear living under an unjust power anymore," said Thantawut Twewarodomgul, an activist who had previously served a jail sentence for royal defamation. He left Thailand for Laos in the wake of the coup.

In Laos, some launched digital radio stations to keep up their opposition to the generals and -- in some cases -- the kingdom's unassailable monarchy.

- 'Shocked and scared' -

But rights groups say their continued activism may have jeopardised their lives.

Three firebrand radio hosts have been reported missing in Laos since the coup, according to Human Rights Watch.

Late last year the corpses of two aides of radio host Surachai Danwattananusorn washed ashore on the Thai-Laos border.

Their disemboweled bodies were stuffed with concrete blocks and their faces battered beyond recognition.

The men lived with Surachai, 77, who had fled to Laos in June 2014 soon after the coup, and hasn't been seen since mid-December.

His wife Pranee, who remains in Thailand, fears the lifelong activist -- who spent nearly three years in jail in Thailand after being sentenced under royal insult laws -- is dead.

"In a democracy, people should be able to criticise public figures," Pranee told AFP.

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The grim case has injected fear into the exiled community.

"We are shocked and scared. We are looking to leave Laos," another exile involved with the anti-junta radio, who declined to be named for their safety, told AFP.

- Activists at risk -

After four years smothering political debate, the junta lifted some of its bans on political activity weeks before announcing elections.

But there is little trust its repressive reflexes have dulled.

Pro-democracy campaigner Nuttaa Mahattana, 39, said she was slapped with "sedition" charges for anti-junta stunts.

"Very few voices have risen in the kingdom against the junta. Someone had to do it," Nuttaa said.

She is among thousands of activists facing prosecution in Thailand -- rights groups say nearly 2,000 have gone to trial in military courts.

These lawsuits are "a reminder of the perils of speaking out," said Tyrell Haberkorn, a researcher on Thai state violence from the University of Wisconsin-Madison.

Prosecutions under a royal defamation law -- referred to as "112" after its criminal code -- spiked early in the junta rule, with record sentences meted out.

It has been upheld as a necessity to protect the monarchy but rights groups decry it as a tool against dissent.

There were no fresh charges in 2018 -- two years after Thailand's new King Maha Vajiralongkorn ascended the throne -- and the halt in cases could signal the palace's shifting attitude regarding 112's use, said a senior police source.

But Haberkorn remains unconvinced the law, which she said was used to "create a climate of fear", is in retreat.

"It will not disappear with elections."

- From streets to online -

Thailand's northeast is the heartland of supporters loyal to the powerful Shinawatra clan, and serves as the home to the "Red Shirts" -- a grassroots pro-democracy movement made up of farmers and the urban working-class.

One of the junta's first moves was to head off any possible Red Shirt resistance, and its leaders have been "summoned, visited and surveilled", a former leader told AFP, requesting anonymity.

"We just don't have the power" to mobilise anymore, he said.

With the streets off-limits to protest for several years, anti-junta sentiment has spilled online, with memes, videos and raps against the military going viral.

But freedom on the web is not a sure bet.

The draconian Computer Crime Act, revised by the junta, has been wielded against online critics.

The junta-picked parliament also passed a cybersecurity bill, allowing authorities to seize devices without court orders when confronted with perceived "critical" threats.

While critics have likened the bill, which allows officials to ask internet providers and users for personal information, to a "Big Brother" mandate, junta spokesman Werachon Sukondhapatipak told AFP it is necessary to "protect citizens against cyber crimes and fraud".

But the prospect of the new law in the hands of a junta eyeing a return to power in civilian clothing has left many queasy.

"If the junta-allied party forms the new government, they will retain the same measures," said Yingcheep Atchanont of monitoring group iLaw.