The investigating officer said the accused, who was allegedly working for an ISI spy ring, was produced before Duty Magistrate Arun Kumar Garg and remanded for custodial interrogation till November 8.
He said Farhat has to be confronted with other arrested men to unearth the larger conspiracy and nab others.
Delhi Police claimed that Farhat was involved in the espionage ring in which Pakistan High Commission staffer Mehmood Akhtar, described as the kingpin working for Pakistan's Inter-Services Intelligence, was detained by police for alleged possession of sensitive defence documents.
It said that Farhat was detained last night and was arrested this afternoon after prolonged questioning.
The officer said that preliminary questioning has led to certain revelations that need to be further investigated as other names have cropped up.
The agency has already arrested three persons -- Shoaib Hasan, Maulana Ramzan and Subhash Jangir -- earlier in the case who are in police custody till November 8.
During the proceedings today, police alleged that more persons are likely to be apprehended and more documents and other evidence to be recovered with the help of the accused.
The agency also said that all the accused persons arrested so far would be confronted with each other and taken to various places in the course of the investigation.