Police arrested a singer-turned-activist suspected of torching the property in front of its Klongprem Central Prison.
Chai-amorn Kaewwiboonpan, better known as Ammy The Bottom Blues, was arrested at an unregistered rented room in Ayutthaya province and he was brought to Bangkok.
The suspect told police he was ill and police brought him to Police General Hospital in Bangkok.
It was reported that Pol Lt Gen Pakapong Pongpetra, commissioner of the Metropolitan Police Bureau, would hold a press conference on the arrest at the bureau at 10.30am.
Mr Chai-amorn was wanted under an arrest warrant for lese majeste and arson in connection with a fire in front of the Klongprem Central Prison in Bangkok in the morning of Feb 28.
He was also charged with computer crime as the fire incident was posted via social media. Police were expanding investigation to other people involved in forwarding the pictures of the arson.