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A man arrested over the bombing of an army hospital in Bangkok said Tuesday that he carried out the attack that wounded 21 people as an act of resistance against Thailand's ruling military junta, his criticisms of the government overshadowing police pride in solving the case.

Wattana Pumret, a 61-year-old retired electrical engineer for a state enterprise, said at a police news conference that he acted on his own when he planted a bomb in a guest reception room in Phramongkutklao Hospital on May 22, the third anniversary of a military coup that ousted a democratically elected government.

In front of senior police officials, he said calmly that he carried out the bombing as a "symbol of defiance" against Thailand's military government.

"I do not hate the army, I love the army. But I hate some soldiers who use the people as a stepping-stone to gain power and become prime minister and the government," said Wattana, who was allowed to answer questions.

Police said they have also charged him for several other bombings as long ago as 2007 for which Wattana took responsibility. No one was killed in any of them. Wattana was charged for four bomb attacks and for unauthorized possession of weapons and explosives.

"I had no intention of causing any harm. I want to deeply apologize for the actions that caused injuries both severe and minor. I really want to apologize," Wattana said. He called the bomb he used "a giant firecracker."

Thailand experienced intermittent but sometimes severe political violence after another coup in 2006, as contending factions battled for power. Wattana said he had attended rallies of both sides — the pro-democracy "Red Shirts" and the royalist "Yellow Shirts," whose activities were backed by the army.

"I am just a citizen who disagrees with governments that come from coups," Wattana said. "Each time there is a coup, it is a massive loss to the country in terms of the economy as well as for the people's rights and liberties, which are stripped away."

Soldiers apprehended Wattana last week and kept him in custody before he was handed over to police. Wattana said that he was treated fairly and given adequate rest and food while he was under military custody. Thailand's military has been criticized by rights groups for its rights abuses to those under military detention.

"While I was under custody by the military, I was treated very well and my basic human rights needs were met," Wattana said. He then thanked by name each soldier who held him captive for their treatment toward him.

Police made a video presentation at the news conference of the evidence against Wattana, including surveillance camera footage.

Security officials earlier had seemed to intimate that they believed there was a connection between Wattana and the Red Shirt movement, which they are keen to discredit.

In an evident effort to dispel the suggestions that he was the paid tool of any organization, Wattana said that each bomb cost him just 50 baht ($1.47) to make.

"I only wanted to send a message to the coup government that the grassroots citizens do not admire governments that come from coups," he said.

Prime Minister Prayuth Chan-ocha, an apparent target of Wattana's criticism, told reporters later Tuesday that Wattana was a "lone wolf" who acted on his own.

"But minor suspects, in our country are often backed by larger figures," he said. "Police investigations will continue and if they find any more suspects involved, they must arrest all of them."