BANGKOK: Thai activists on Monday demanded the release of a man arrested for sharing an infographic on Facebook detailing alleged graft in an army-built park, saying plainclothes security officers took him by force.

Since taking power in a military coup in May 2014, Thailand's ruling junta has issued directives that have largely stifled dissent, including barring political discussions and debate.

On Sunday, a 25-year-old student, Thanet Anantawong, was taken from a hospital while he awaited an operation, said prominent anti-junta activist, Siriwat Serithiwat.

"Plain clothes security officers went to a hospital where Thanet was staying," Siriwat told reporters outside a criminal court in the capital. "I would like the court to release Thanet. He needs medical attention. We are afraid for his life."

A junta spokesman declined to comment on the arrest of Thanet, who is being held at Bangkok's 11th Army Circle military base.

Thanet was among a group of activists who tried to visit the park, at the centre of a corruption scandal that threatens to embroil the military government.

Soldiers and police intercepted them at a train station, and detained some for several hours before their release.

The Rajabakti park, built in the seaside resort town of Hua Hin south of Bangkok, and dedicated to the monarchy, has been at the centre of allegations of corruption and misspent funds.

A military probe into its finances found no corruption, but graft accusations persist among opposition groups and the media.

Thanet faces charges under Article 116 of Thailand's criminal code - the equivalent of sedition - as well as under the wide-ranging Computer Crimes Act for allegedly re-posting a diagram on Facebook linking junta leader Prayuth Chan-ocha and other officials to alleged corruption involving the park.

A second man, Thanakorn Siripaiboon, 27, was arrested on Dec. 8 for sharing posts about government corruption and hitting the "like" button on a post with an image deemed insulting to Thailand's king, Human Rights Watch said on Saturday.

He is being held at Bangkok's high-security Klong Prem Central prison, police said.

The junta has made a top priority of protecting the monarchy through the use of Thailand's royal defamation laws, which are among the world's harshest.

Those accused of lese-majeste are tried in military courts, which have handed down record sentences.

A military court refused Thanakorn bail, said his lawyer, Anon Nampa.

"They said his crimes are punishable by many years in prison and they are afraid he will offend again if released," he added.