Investigators found remnants of batteries and wires at the scene of the blast on the ground floor of Phramongkutklao Hospital, said Srivara Ransibrahmanakul, the deputy national police chief.
“We can confirm at this stage that it is a bomb,” he said.
Three people received serious shrapnel injuries to the face and neck, but most of the victims had minor wounds, said Sansern Kaewkumnerd, a government spokesman.
It was not immediately clear who was behind the explosion or if it was connected to the anniversary of the 2014 military coup that overthrew a democratically elected government.
“Please don’t connect this with any other incidents because they may or may not be connected,” Sansern said. “We have to wait for officials to investigate details from surveillance cameras in the area.”
Phramongkutklao is a military-run hospital that is also open to civilians. The blast radius from the explosion was 2 to 3 meters (up to 9 feet), police said.
No Australians have been injured in the blast.
A Department of Foreign Affairs and Trade spokesman confirmed just after 6pm AEST that the Australian Embassy in Bangkok had contacted the Phramongkutklao (King Mongkut Hospital) Hospital in the Rajthevi District of Bangkok regarding the explosion on site which injured more than 20 people.
“The hospital has advised that no Australians have been affected,” the spokesman said.
DFAT has advised any Australians concerned for friends and family to attempt to contact them directly.