An officer in the Delhi Police was killed by a man outside the city police station on Saturday morning.
The victim was identified as Dharamshala police constable Ramkumar.
Sources say the man had threatened the constable and his family before the attack.
Police sources said the constables mother and father were present when the attack took place.
Police said the man was shot and later succumbed to his injuries.
The incident took place on the outskirts of the city.
Police officials said the suspect was arrested at his home in Rajasthan.
They said they have registered a case under section 307-A of the IPC and have sent a team to probe the incident.