Mr Geelani, the police said, was booked as he is supposed to be the "main organiser" of the event.
Former Delhi University lecturer SAR Geelani -- who was acquitted in the Parliament attack case - has been detained by the police for allegedly organised a programme at the Press Club in support of Afzal Guru. A case of sedition was filed against him on Friday.
"Mr Geelani has is being questioned at the Parliament Street Police Station," a senior police official said. The police said they registered the case after taking note of media clips of the incident.
The detention comes amid the raging controversy over the arrest of Kanhaiya Kumar, the president of the student's union of Jawaharlal Nehru University, under sedition charges.
He was arrested after an event was held in the university campus in support of executed Parliament attack convict Afzal Guru on February 9, during which anti-India slogans were raised.
It triggered a countrywide students' protest and the opposition parties have accused the government of overreaction on something that could have been handled within the institution.
On February 10, a group allegedly shouted slogans hailing Guru at the Delhi Press Club, following which the police registered a case of sedition, criminal conspiracy and unlawful assembly against Mr Geelani and unnamed persons.
Mr Geelani, the police said, was booked as he is supposed to be the "main organiser" of the event.
"The request for booking a hall at the Press Club was done through Geelani's e-mail and the event was proposed to be a public meeting, which did not turn out to be so," a senior police officer said.
After a case of registered, the police questioned DU professor Ali Javed, a Press Club member, who had booked the hall for the event for two consecutive days.
Mr Geelani was arrested in connection with the 2001 Parliament attack case. But two years later, he was acquitted by the high court for want of evidence. In August 2005, the decision was upheld by the Supreme Court, which, however, said that the "needle of suspicion" pointed towards him.