A bomb explosion at a hospital in Bangkok has injured 21 people, Thai police say.

The blast at Phramongkutklao Hospital, which is popular with retired military officers, happened three years to the day since the military took power in a coup.

Investigators have found traces of batteries and wires at the scene - and the device was "packed with a large number of nails".

The bomb had been hidden in a container near the hospital's pharmacy, sparking panic and sending smoke into the corridors.

Most of the wounded were hit by flying glass.


Image: Forensic police carry evidence away from the scene of the explosion in Bangkok

A government spokesman said one woman had been seriously injured, and needed surgery because of shrapnel buried in her jaw.

Most of the victims had minor injuries, and 13 of those hurt were discharged soon after the blast.

Commander Kamthorn Aucharoen said: "It was a bomb. We found the pieces that were used to make the bomb.

"Right now, authorities are checking out closed circuit cameras."

Although owned by the military, Phramongkutklao Hospital is open to civilian patients.

Human Rights Watch described the attack as a "cruel and inhumane action which grossly violates human rights".

Image: The hospital attack came on the third anniversary of a military coup in Thailand

It is not known who was responsible for the explosion, but suspicion is likely going to focus on political dissidents opposed to military rule - or Muslim separatists based in the south of the predominately Buddhist nation.

Earlier this month, a car bomb at a shopping centre near Thailand's border with Malaysia wounded 61 people.

Authorities blamed that attack on Muslim insurgents.