The incident took place last evening and the deceased has been identified as Raja, a labourer working at a tannery in Jajmau in Chakeri area.
Based on a tip-off, a police team was sent to the spot, Station Officer Chakeri police station said.
When they saw the police approaching, Raja and two of his friends jumped into the river. The other two were nabbed by the police when they swam ashore but Raja did not surface for a long time.
Meanwhile, Raja's relatives created a commotion upon reaching the spot. A relative, Waseen, alleged that the police had conducted the raid on receiving a tip-off about Raja and his friends from other gamblers. He also said the police did not try searching for the deceased for a long time after he had disappeared underwater.
Police officials have ordered an investigation into the incident.