Kumar was invited to speak about his book "From Bihar to Tihar" at the event at Sheroes Cafe, run by acid attack survivors, on the first day of the festival.
On learning about Kumar's presence at the event, some people, allegedly belonging to right-wing groups, reached the venue and raised slogans against him.
Kumar's supporters were allegedly jostled by the protesters as slogans of "Bharat Mata ki jai" and "Vande Mataram" rang out.
The organiser, Shamim Aarzo, said, "Actor Divya Dutta was our guest for the first session last night. Thereafter, Kumar was to speak about his book... that is when about a dozen activists belonging to some organisations started creating disturbance."
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"We tried to placate them... police were called in to bring the situation under control. Later, the session with actor-turned-politician Shatrughan Sinha was taken up according to the schedule," Aarzo said.
Those present in the audience claimed the protesters belonged to some right-wing groups.
When contacted, District Magistrate Kaushal Raj Sharma, said the organiser had taken permission only for holding a book fair and the administration was not told that leaders or actors will be there or deliver any speech at the event.
"The Model Code of Conduct is in force in view of the upcoming local body elections and because we had no knowledge that leaders and actors had been invited to the festival, security arrangements were not made," he said.
However, the organiser said they had informed the administration about the event and only Dutta had confirmed her participation by that time.
"Kumar's participation was confirmed later... His event was publicised and it was not a secret. The session with AIMIM MP Asaduddin Owaisi will go live on Facebook today," Aarzo said.