Police arrested singer Chai-amorn Kaewwiboonpan, aka Ammy The Bottom Blues, in Ayutthaya province early on Wednesday for allegedly setting fire to a portrait of the King.
Pol Maj Gen Piya Tawichai, deputy commissioner of the Metropolitan Police Bureau, said Mr Chai-amorn was taken into custody at a rented room about 12.40am. The singer cited illness and requested he be transferred to a hospital.
Police were taking him to a hospital in Bangkok, he said.
Police on Tuesday denied reports that the singer had been arrested.
The deputy commissioner said legal action would also be taken against other people, after a video of the incident was shared around.
Bangkok police chief Pol Lt Gen Pakapong Pongpetra later told reporters that Mr Chai-amorn had earlier suffered an injury to his left shoulder and hip. Police took him to Rajthanee Hospital for initial treatment, and then to Police General Hospital in Bangkok.
He said police had witnesses and forensic evidence to charge Mr Chai-amorn with participating in the portrait burning. The singer was the only suspect in the case arrested so far. He had not admitted to the crime, Pol Lt Gen Pakapong said.
On Tuesday the Criminal Court issued arrest warrants for suspects in the burning of a portrait of His Majesty the King in front of the Klongprem Central Prison in Bangkok in the early hours of Sunday.
Mr Chai-amorn and two others are facing charges of arson, royal insult and computer crime.
The charges carry severe penalties — 5-20 years, life in prison or death for arson; 3-15 years for royal insult; and five years each for trespassing on a state office and for computer crime.