Twenty-five people were wounded in the blast at the Phramongkutklao Hospital near Bangkok's historic Victory Monument, which is popular with retired military officers.

"It's confirmed to be a bomb," said Bangkok police commissioner Srivara Ransibrahmanakul.

Traces of a battery and wires were found among the debris.

Most of those hurt were hit by flying glass.

Four retired military officers were in the reception room at the time of the blast, awaiting appointments with doctors, hospital staff said.

The list of the injured issued late on Monday contained 25 names, 12 of whom had army ranks. Three of the injured require close monitoring. Police said the blast emitted smoke but no flames.

Police and soldiers had increased security across the country, fearing attacks to mark the anniversary of the coup that toppled a democratically elected government, after months of political instability.

Two weeks ago a small bomb exploded in front of Bangkok's National Theatre, wounding two people.

On May 9 at least 56 people, including children, were wounded when two huge bombs exploded at a busy supermarket in Thailand's south where Muslim militants have been waging a bloody insurgency.

Attacks in the southern provinces of Pattani, Narathiwat and Yala have escalated since mid-April, when Thailand's military government rejected the terms of new peace talks proposed by the Barisan Revolusi Nasional, one of a number of insurgent groups.

A key demand was that talks include international negotiators and observers.

The military has dominated civilian life in Thailand since 2014, stacking parliament with military officers, quickly quashing any sign of dissent and censoring the media.

The military has detained hundreds of people and held them in military camps for so-called "attitude adjustments."

Prime Minister Prayuth Chan-ocha, who led the coup, has promised elections in 2018 but has not set a date.