The incident occurred this morning when Mannan was erecting a boundary wall over a disputed plot along with his three sons, SP (City) Ram Mohan Singh said.
A person identified as Zamir reached the spot with a few other men and had a heated exchange of words with Mannan over erection of the wall.
A scuffle broke and one of Zamir's supporters opened fire in which Mannan died on the spot while three others were seriously injured, Singh said.
An FIR has been registered by Mannan's brother Bilal, naming Zamir and three others, who are absconding, he said.