By Amy Sawitta Lefevre
BANGKOK (Reuters) - A group of Thai lawyers called on Wednesday for an investigation into allegations that four suspects held over a Bangkok bomb attack were tortured while in police custody.
The call came amid growing concern about tough action by the military government to stifle dissent under martial law, which has been in force since a military coup in May.
A small bomb went off last month outside a court in the capital, Bangkok, causing minor damage. It followed similar small, twin explosions outside a shopping mall on Feb. 1.
The blasts are widely believed by the public to signal dissatisfaction with military rule and they raised questions about the durability of an uneasy calm since the coup.
Police said 13 people were arrested in connection with the blast at the court and the group Thai Lawyers for Human Rights said four of them, all men, were tortured while in police custody.
"The suspects were kicked in the back ... Some said they were electrocuted during questioning between March 9 and March 15," the group said in a statement.
Police denied the men were tortured.
Among the 13 arrested was a volunteer nurse, Natthida Miwanpa. She had earlier said that she witnessed the shooting of civilians by soldiers during violent political confrontations in 2010.
Police said on Tuesday Natthida was charged with terrorism and was being held at a Bangkok prison.
But her family said she was missing for six days before that and New York-based Human Rights Watch group accused the military of holding her in secret.
"Holding a witness to alleged military crimes incommunicado for six days is a profoundly disturbing abuse of authority that has become commonplace under martial law," said Brad Adams, the group's Asia director.
The ruling junta, known as the National Council Police and Order, denied holding her.
Junta spokesman Colonel Winthai Suvaree also denied that the military was using martial law to go after its critics.
"The government is not trying to go after those who disagree with its views," he said. "Only those who cause extreme damage to national security will be kept under military custody."
(Additional reporting by Panarat Thepgumpanat and Aukkarapon Niyomyat; Editing by Robert Birsel)