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.