The deceased has been identified as Munna Lal (30) who hailed from Bihar, they said.
The incident occurred today morning when the labourers were working at a construction site of Jaipur Development Authority near Science Park in the city.
Four cranes and two JCB machines were pressed into service and the body was recovered after around seven hours, said police.
The construction site was very delicate as it leads to Aminashah Nullah and any further damage to the mud or concrete can be risky for the trapped labourer and entire bridge area, they added.