The incident took place at around 10:30pm last night when constable Harish was patrolling in Valmiki Colony area.
Police got a phone call from a woman, identified as Guddi, at 10:15 PM stating that three men are beating her up. Following the call, the constable was asked to check, said a police official.
As Harish reached the spot, he saw three men beating up a woman. Seeing him, the trio tried to flee but the constable managed to nab one of them. Seeing their associate being caught, they attacked the constable with bricks, one of which hit him on the head making him unconscious, he said.
"Guddi wanted to put up a tin shed on her house to save herself from rains. The three men, who were local criminals, were demanding money in order to allow her put the shed", the official said.
A case has been registered at Malviya Nagar police station and the three accused have been identified but they are yet to be arrested, police said.