Thailand remains starkly divided since the May 22, 2014 coup, but dissent has broadly been smothered by a military with sweeping security powers.
While it was not immediately clear who was behind the blast, Thailand has a long history of bomb attacks on symbolic dates carried out by militant political factions or separatists linked to an insurgency in the Muslim-majority south.
Monday's blast struck near the VIP section of the King Mongkut hospital as patients and their families waited for prescriptions, shattering glass and sending smoke into the corridors.
Hospital director Saroj Keokajee, said the "low intensity bomb" injured 21 people, among them retired military officers.
"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," he said.
The clinic in central Bangkok is often used by serving and retired members of the armed forces.
But Saroj said no senior military officers were near the blast which hit the 'Wongsuwan Room' -- the Thai junta number two is called Prawit Wongsuwan.
A battery and wires had been found at the scene, Deputy National Police Chief General Srivara Rangsibrahmanakul told reporters.
The bomb "was likely to be in a package", Srivara said.
Regardless of the motive, the blast will raise the political temperature in Thailand where violence had declined under the military's stranglehold.
Police are already hunting suspects behind two other small blasts in recent weeks, but have given conflicting and contradictory information over the devices and likely suspects.
Forced stability
Despite a veneer of stability Thais remain divided and uncertain over the future three years after the fall of the elected government of Yingluck Shinawatra.
Protest and political gatherings are banned while dissidents have been rounded up on charges of sedition or breaching junta orders or under the draconian royal defamation legislation.
Militant elements among pro-democracy groups have either been arrested or gone to ground.
The one region where daily violence and large bomb blasts persist is the country's "Deep South" where Malay Muslim militants have fought a long insurgency.
But they rarely strike outside their region -- an exception being in August 2016 when a series of coordinated blasts hit a string of tourist towns.
The country's notoriously fractious domestic politics have incubated the worst violence.
Over the past 10 years Thais have witnessed repeated rounds of deadly protests, a string of short-lived governments and two military coups that deposed elected leaders.
The junta says its 2014 coup -- the 12th time generals have successfully seized power -- was needed to bring stability and root out corruption.
But critics say the military is deeply hostile to ousted premiers Thaksin Shinawatra and his sister Yingluck, whose parties have won every poll since 2001.
Their billionaire clan is popular among Thailand's rural and urban poor and they have urged a return to elections.
But the Shinawatras are hated by Bangkok's military-backed elite, who accuse the family of corruption and nepotism.
In a statement on Facebook to mark the coup Yingluck decried a lack of "concrete reform" and warned that three years of military rule risked becoming a "waste of time".
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