Samajwadi Party's Rajya Sabha MP Munabbar Saleem 's personal assistant Farhat was held by the Crime Branch in connection with the espionage racket in which three persons have already been arrested.
A senior Crime Branch officer said Farhat's interrogation is underway.
Mehmood Akhtar, a visa officer at the Pakistan High Commission, was arrested on Wednesday. However, he was released hours later due to diplomatic immunity.
In a statement, Akhtar had revealed the name of Farhat. Reports said that Farhat had provided government documents to the Pakistani spy.
Meanwhile, the Samajwadi Party has said that it will seek an explanation from the MP on the matter.
Akhtar, detained on Wednesday and let off because of diplomatic immunity, had served in the mission for over two years during which he recruited Indians to spy for Pakistan's Inter-Services Intelligence, Delhi Police Joint Commissioner Ravindra Yadav said on Thursday.
He was caught near the Delhi Zoo receiving defence-related vital information from two Indian nationals, Maulana Ramzan and Subhash Jangir, who were also allegedly working for the ISI, Yadav said.
One more person, Shoaib Nagaur, was arrested in Jodhpur on Thursday evening. The three are residents of Rajasthan and were being interrogated.