Purushottam Rana, against whom a lookout notice had been issued, was apprehended from outside Patiala Court complex when he allegedly came to surrender.
"We have arrested him," Ravindra Yadav, Additional Commissioner of Police (Crime), told PTI.
BSP leader Deepak Bhardwaj, who was also the richest candidate in the last Lok Sabha elections, was shot dead allegedly by Rana and two other men at his farmhouse in south Delhi's Rajokri on March 26.
Investigators had yesterday detained a person suspected to have driven the car in which the killers fled from the farmhouse.
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Bhardwaj's wife Ramesh Kumari, his sons and his staff have been questioned by the police in the past few days, with investigators saying that property or money were plausible reasons for the murder.
Investigators are also looking into the role of an insider who might have tipped off the killers about the movement of Bhardwaj and the topography of the farmhouse.
Around 40 employees were inside the 25 acres farmhouse at the time of the incident. However, his two Personal Security Officers were not with Bhardwaj or in the farmhouse then.