Thai authorities said a number of Chinese tourists were among the victims of the shrine blast.
Defense Minister Prawit Wongsuwan said authorities had no idea an attack was planned. "We didn't know about this ahead of time. We had no intelligence on this attack," he said.
Prayuth vowed to "hurry and find the bombers," though he said there may be just one perpetrator. Speaking to reporters, he continued what has been a notoriously prickly relationship with the media since he took control of the government in a May 2014 coup.
Asked if there were leads on the suspects' identities, Prayuth, a former general, bristled, saying, "We are still investigating. The bomb has just exploded. Why are you asking now? Do you understand the word 'investigation'? It's not as if they claim responsibility."
Thailand has seen many attacks in recent years, particularly related to an insurgency more than a decade old by Muslim separatists in the country's far south that has left more than 5,000 people dead. But those attacks never extended to the capital or were on the scale of Monday’s attack.
Bangkok has seen politically charged violence over the past decade as well. The deadliest, in 2010, saw more than 90 fatalities over two months and was centered on the same intersection where Monday's bomb went off. But none of those assaults included a bombing apparently intended to produce mass casualties.
Matthew Wheeler, a Southeast Asia security analyst for the International Crisis Group, said the explosion was a "new type of attack for Bangkok" that doesn't bear the trademarks of typical violence over the past decade from political instability or Muslim separatists.
"It is certainly not like politically motivated attacks we've seen in the past, which have generally been designed to grab attention but not cause casualties," he said, adding that he expected it would have "major ramifications for security in Thailand."
Bangkok has been relatively peaceful since a military coup ousted a civilian government in May last year after several months of sometimes violent political protests against the previous government.
At the same time, the military government has tightly controlled dissent, arresting hundreds of its opponents and banning protests. Tensions have risen in recent months, with the junta making clear that it may not hold elections until 2017 and wants a constitution that will allow some type of emergency rule to take the place of an elected government.
Still commanding a wide following in the country is exiled former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 coup. His sister, Yingluck Shinawatra, was deposed last year.
Last week Thaksin posted a message on YouTube urging his followers to reject the draft constitution because it is undemocratic. The draft charter is supposed to be voted on next month by a special national reform council. If the draft passes, it is supposed to go to a national referendum around January.
Al Jazeera and The Associated Press