The explosion occured at a garbage dump at Sarbamangala Ghat in the Sutanati slum area in Chitpur, mainly inhabited by ragpickers.
"Four people were killed and nine others were injured after a crude bomb exploded in garbage dump. The incident happened when some ragpickers went to collect plastic pouches from the dump," additional commissioner of police Subrata Narayan Sarkar told PTI.
"As they were scourging through the dumping ground they found the bomb and ignorantly tried to break it and that caused the explosion," Sarkar added.
The dead were identified as three-year-old Saket Sana, Ranjan Mollah, Hasan Sardar and Didar Mollah. They were all ragpickers and lived in the slum.
All the injured have been rushed to the nearby R G Kar Medical College and Hospital.
According to hospital sources, the condition of three is serious.
So far it seems to be a one-off case and an accident and not a carefully planned operation, a senior police officer said. The bomb squad and forensic department have been summoned and they were trying to find out whether there was a chemical reaction which has caused the explosion.