He saw six bodies under sheets inside the shrine and one outside of it, he told CNN.
The bomb that detonated just after 7 p.m. was fastened to a utility pole in front of the shrine, and the bomb squad was working to deactivate a second device about 50 minutes after the first blast, the Bangkok Post reported.
The explosion, which occurred near the Erawan Shrine, a Hindu place of worship and tourist attraction, destroyed several motorcycles.
Though Buddhism is the predominant religion, there are many Hindu shrines in Bangkok, and the Erawan Shrine is the best known among them, according to an English-language website providing news and tourism information about the country.
The shrine sits at a busy intersection with the city's Skytrain rumbling nearly overhead. Many garland sellers line the sidewalk nearby, according to the website.
There also are numerous shopping centers in the area, Google Maps shows, including the Platinum Fashion Mall and Central World, reportedly one of the largest shopping malls in the world.