The attack took place close to the Erawan shrine in Bangkok’s central Chidlom district.
The shrine is a major tourist attraction. The Thai government said the attack was aimed at foreigners.
Local media report that tourists may be among the casualties.
Defense Minister Prawit Wongsuwong said: “It was a TNT bomb… the people who did it targeted foreigners and to damage tourism and the economy.”
The Nation TV channel quoted PM Prayuth Chan-ocha as saying the government would set up a “war room” to co-ordinate its response.
The explosion occurred at about 19:00 local time. Some reports said the bomb had been on a motorcycle, others that it was tied to an electricity pole.
The shrine is to the Hindu god Brahma, but is also visited by thousands of Buddhists each day. There are also three major shopping centers nearby.
National police spokesman Lt Gen Prawut Thavornsiri told Agence France-Presse news agency: “I can confirm it was a bomb, we can’t tell which kind yet, we are checking.”
The explosion was on the Ratchaprasong intersection, which has been the center of political demonstrations in recent years.
Bomb attacks in Bangkok are extremely rare.
There has been a Muslim insurgency, but this has been largely confined to the south of the country and attacks rarely take place elsewhere.
However, Bangkok has seen a decade of sometimes violent rivalry between political factions.
The military took over ruling the country in May last year, removing an elected government following months of unrest.
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