Image copyright EPA Image caption The bomb blast happened in a waiting room inside a military hospital

More than 20 people have been injured in a small bomb blast at a military hospital in the Thai capital, Bangkok.

The explosion happened at Phramongkutklao Hospital, which is used by serving and retired members of the military and their families.

It took place on the third anniversary of the 2014 coup which brought the military government to power.

Police said it was not clear who was behind the blast, which happened in a waiting room by a pharmacy.

"We found pieces of a circuit and a battery, as well as some wires. We'll investigate further but initially we found the explosive had a range of 2-3 metres," the Bangkok Post quoted Deputy National Police Chief General Srivara Rangsibrahmanakul as saying.

Officials said most people had minor wounds from flying glass.

"Eight people were admitted to hospital to observe their condition... among them is one woman who needed surgery because of shrapnel buried in her jaw," AFP quoted hospital director Saroj Keokajee as saying.

Thai newspaper The Nation said Prime Minister Prayuth Chan-ocha had ordered an immediate investigation into the attack at the hospital, which also treats civilians.

Political divisions - Jonathan Head, BBC News, Bangkok

Three years after the armed forces seized power in Thailand, they rule almost unchallenged; the few who have dared oppose the government openly have been detained and charged.

But there have been occasional small bombs, presumed to be in protest against the military. One went off last week close to the royal palace, where preparations are still under way for the elaborate cremation of the late King Bhumibol, who died last October.

The military has succeeded in maintaining order and stability through a sensitive royal succession; but it has done little to address the political divisions underlying the conflicts which preceded the coup.

Three years ago to the day the military ousted an elected government following months of street protests.

Since then, Thailand's military leaders have clamped down on critics, political opponents and free speech.

An election is due by the end of next year. It will be held under a new military-drafted constitution that critics claim only allows for a partial, guided democracy.

Thailand has faced a power struggle since Thaksin Shinawatra was ousted by the military as prime minister in 2006.

He and his sister (who led the government ousted in 2014) have strong support in rural areas, but they are hated by a military-backed urban elite who accuse them of corruption.