The gun was seized when the man, Sreekumar was trying to sell it to a prospective customer near a bridge, police said.
The Organised Crime Investigation Unit of Police was monitoring his movement for the last few days, following the information disclosed by one Soman, who was arrested for illegally manufacturing guns in his house some two months ago, about his customers, police said.
Seeing the police, Sreekumar attempted to throw the gun in the river nearby, but the alert sleuths caught him along with the weapon, police said.