Five died on the spot in the explosion at a junk recycling shop about 12.50pm. The blast demolished the building and started a large fire. Body parts were found as far as 200 metres from the scene.
Twenty more people were reported injured by the Erawan medical centre.
The blast was caused by a 500-pound World War II (227kg) bomb, the reports said.
Before the explosion, a construction worker at a site near the shop found a heavy piece of iron during piling work.
An employee at the shop also did not know it was a bomb. He therefore tried to cut it open with a gas torch, sparking the explosion.
The bomb could have come in with the soil used to fill the site before construction began.
Police said the explosion left a crater four metres wide and three metres deep where the shop once stood.
Several nearby houses were also damaged by the blast, they said.