"Geelani has been detained and he is being questioned at the Parliament Street Police Station," a senior police official said.
His detention comes amid the raging row over the arrest of JNU students' union president Kanhaiya Kumar over sedition charges in connection with an event on the varsity's campus against the hanging of Parliament attack convict Afzal Guru on February 9.
At the Press Club event on February 10, a group allegedly shouted slogans hailing Guru, following which the police had registered a case under sections 124A (sedition), 120B (criminal conspiracy) and 149 (unlawful assembly) of the IPC against Geelani and other unnamed persons.
The police had claimed to have registered the FIR taking suo motu cognisance of media clips of the incident.
Police also claimed that Geelani was booked as he is presumed to be the "main organiser" of the event.
"Request for booking a hall at the Press Club was done through Geelani's e-mail and the nature of the event was proposed to be a public meeting, which did not turn out to be so," a senior official had said.
Following the registration of the FIR, the police questioned for two consecutive days DU professor Ali Javed, a Press Club member, who had booked the hall for the event.
Geelani was arrested in connection with the 2001 Parliament attack case but acquitted for "need of evidence" by the Delhi High Court in October 2003, a decision upheld by the Supreme Court in August 2005, which at the same time had observed that the needle of suspicion pointed towards him.
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