The youth, identified as Rajesh Kumar, suffered bullet injury in his neck in the attack. He underwent am emergency surgery at AIIMS Trauma Centre, where his condition is stated to be critical, police said.
Kumar's father Bhagan is a dairy owner and supplies milk to Mother Dairy booths in west Delhi.
"The father-son duo had gone to Paschim Puri Pocket-1 to supply milk at a booth around 5:30 AM when three men on a bike intercepted Bhagan and told him to handover cash while his son had gone to the booth. Bhagan told them that he was not carrying any money," a police official said.
Meanwhile, Rajesh arrived at the spot and seeing his father being hit, took on the assailants and tried to overpower them, he said.
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Amid the scuffle, one of the accused opened fire and a bullet hit Rajesh in his neck. The trio then fled from the spot on their motorcycle, leaving behind their pistol.
Someone informed the police which took Rajesh to Deen Dayal Upadhyay Hospital, from where he was referred to AIIMS Trauma Centre.