The Nation TV channel quoted Prime Minister Prayuth Chan-ocha as saying the government would set up a "war room" to co-ordinate its response.
National police chief Somyot Poompummuang said that 10 Thais had been confirmed dead, along with one Chinese and one Filipino.
He said: "Whoever planted this bomb is cruel and aimed to kill. Planting a bomb there means they want to see a lot of people dead."
Thai police said 123 people had been injured. The government in Hong Kong said three of its residents were among those wounded.
The explosion occurred at about 19:00 local time (12:00 GMT).