BANGKOK (Reuters) - "Is she dead yet?"

Thai opposition activist Kritsuda Khunasen said she was blindfolded and struggling to regain consciousness when she heard this chilling query from one of her interrogators.

She was detained for nearly a month by the Thai military after it toppled the government of Prime Minister Yingluck Shinawatra on May 22. Kritsuda said she was beaten by soldiers and hooded with a plastic bag until she blacked out.

"That was the moment I thought I'd died," she told Reuters via Skype from a secret location in Europe, where she is seeking political asylum.

Junta chief General Prayuth Chan-ocha has dismissed Kritsuda's claims as "totally untrue" and ignored United Nations calls for an investigation.

But allegations of abuse in military custody, plus signs of defiance on Thai campuses, undermine Prayuth's claim that the junta is - to borrow the title of his Friday-night TV address to the nation - "returning happiness to the people".

Prayuth has sought to reassure tourists, foreign investors and fellow Thais that the military has restored calm after months of divisive and sometimes deadly street protests.

But anti-coup activists and human rights monitors say that calm is sustained by a climate of fear, selectively but ruthlessly applied against opponents of a military eager to avoid its past mistakes.

Its 2006 coup, which overthrew the protest-besieged government of telecoms billionaire Thaksin Shinawatra, led only to further unrest that culminated in the military's bloody crackdown on Thaksin's "red shirt" supporters in 2010. His sister Yingluck was elected by a landslide the following year.

This time round, the junta - formally called the National Council for Peace and Order (NCPO) - has worked systematically to snuff out all challenges to its authority, no matter how small, said Sunai Phasuk, senior researcher for the New York-based Human Rights Watch.

"They don't blink," he said. "They have power and they use it. They don't care about criticism from the outside world or human rights groups."

The NCPO has detained hundreds of activists, academics, journalists and politicians, and set about purging the bureaucracy of Thaksin sympathizers.

Red shirt leaders told Reuters their movements were monitored, their phones tapped and their families harassed.

"We are very afraid. Most of our members have gone into hiding or are laying low," said a prominent red shirt activist on condition of anonymity. "There are people lurking outside our children's schools."

The junta has silenced other political opponents by publicly threatening to seize their assets. Dissenters living abroad have had their Thai passports revoked.

The military has also shut down websites and exploited draconian laws that forbid all criticism of Thailand's royal family. Martial law remains in place nationwide.

"People who disagree with the coup still live in fear," said Human Right Watch's Sunai. He believed a long period of oppressive military rule could backfire by triggering the very unrest it was designed to suppress.

"Then there will be a very serious threat of violent confrontation," he said.