"It's very bad. Even in war, hospitals are exempt from attacks. This time most of the victims were elderly and patients," said army commander Chalermchai Sittisart.

A pipe bomb left in a flower box ripped through a waiting room for patients at the Phramongkutklao Hospital, injefuse to be sucked into a harmful discourse of 'us and them' that only perpetuates the vicious cycle of hatred that leads to these attacksuring 22 people, three of them seriously, on Monday, the third anniversary of the military seizing power in a 2014 coup.

The hospital is popular with soldiers, their families and retired military officers.

Thirteen of the injured were retired military officers.

Most of the injured were hit by flying glass.

The bomb packed with nails to inflict maximum casualties was left in the hospital's "Wongsuwon" room which is named after deputy prime minister Prawit Wongsuwon, who said even militants waging a bloody insurgency in Thailand's southernmost provinces have not targeted hospitals.

The room serves as a one-stop service centre for commissioned officers and their families.

Police have linked the bomb to two other explosions in the past two months, including one that injured 15 people outside Bangkok's National Theatre on May 15.

Police said the blasts appear to be politically motivated although no group has claimed responsibility.

Red Shirt opposition party leader Jatupron Prompan described the attack as barbaric.

"We are not afraid of being implicated because we had nothing to do with it," he said.

National Human Rights Commissioner Angkana Neelapajit said if the attack was planned to mark the anniversary of the coup it should be regarded as an unspeakable act of terror because a hospital should not be targeted in conflict.

She said although the hospital is run by the army "it treats everyone without discrimination."

The bombings appear to be the first violent challenge to the military that has dominated civilian life in Thailand since 2014, stacking parliament with military officers, quickly quashing any sign of dissent and censoring the media.

Hundreds of Thai students, activists and journalists have been held in military camps for so-called "attitude adjustments."

The military has acknowledged it wants to weaken political parties and maintain a permanent influence over elected governments, partly through a new constitution approved by the king last month.

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

He called for calm after Monday's explosion and ordered increased security across the country.

Thailand has been on edge since May 9 when 61 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 the government rejected the terms of new peace talks proposed by the Barisan Revolusi Nasional, the main insurgent group.

A bomb exploded in Yala on Monday morning, wounding military officers.

Authorities last year accused the insurgents of being behind a series of bomb attacks at Thai beach resorts popular with foreign tourists that killed several people and injured dozens.

More than 6000 people have been killed in the southern conflict over a decade.