A bomb exploded at a popular shrine in central Bangkok during evening rush hour, killing at least 18 people and injuring more than 100, officials said.
No one immediately claimed responsibility for the blast, which caused the worst carnage of any single attack in recent memory in the Thai capital.
The explosion left body parts strewn across the streets of a neighbourhood full of five-star hotels and upscale shopping centres, the officials added.
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.
The area around Bangkok's Erawan Shrine is filled with hundreds of tourists, office workers and shoppers at any given time. Police said the bomb was made from a pipe wrapped in cloth.
"Whoever planted this bomb is cruel and aimed to kill," said national police chief Somyot Poompummuang. "Planting a bomb there means they want to see a lot of people dead."
The shrine is at a major intersection that was the centre of many contentious political demonstrations in recent years - raising questions about whether the bombing was politically linked. But police said it was too soon to determine the attack's motive.
Security video showed a powerful flash as the bomb exploded at around 7pm local time.
At least 18 people were confirmed dead and 117 injured, according to the Narinthorn emergency medical rescue centre.
The dead included Chinese and a Filipino, Mr Somyot said.
Anusit Kunakorn, secretary of the National Security Council, said prime minister Prayuth Chan-ocha, the former army chief who orchestrated the May 2014 coup, was closely monitoring the situation.
"We still don't know for sure who did this and why," deputy prime minister Prawit Wongsuwon told reporters. "We are not sure if it is politically motivated, but they aim to harm our economy and we will hunt them down."
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