The bomb is believed to have been on a motorcycle.
According to a BBC correspondent, there is a huge amount of chaos, with body parts scattered everywhere.
The explosion occurred close to the Erawan shrine in Bangkok's central Chidlom district. According to police, the cause could have been a bomb fixed to a motorcycle.
According to the correspondent, the shrine is a major tourist attraction next to a five-star hotel.
As Thai media report, a second bomb was found in the area and made safe.
According to Thailand's rescue unit, police secured the site. More than 30 cars were damaged by the blast.