A bomb struck a Bangkok military hospital on Monday wounding more than 20 people, police said, three years to the day since the army seized power of the politically unstable kingdom.

"From the initial report it was a bomb... there are more than 20 people injured," Deputy National Police Chief General Srivara Rangsibrahmanakul told reporters.

"It was likely to be in a package," Srivara said, adding that "battery and wires" were found at the scene.

Local media said the device detonated in the hospital pharmacy, sparking panic and sending smoke into the corridors but caused only minor injuries.

Police said the hospital was not evacuated while medical emergency staff at the Erawan Rescue Centre said 24 people were injured.

Police are already hunting suspects behind two other small blasts in recent weeks, but have given conflicting information over the devices and likely suspects.

Thailand remains starkly divided since the May 22, 2014 coup, but dissent has broadly been silenced by a military with sweeping security powers.

Until Monday's blast political violence that defined much of the last decade had subsided, with militant elements among pro-democracy groups arrested or in hiding.