Swaraj Abhiyan chief Yogendra Yadav was allegedly manhandled outside the Jawaharlal Nehru University campus where a clash broke out between members of the students' union and the ABVP.
Yadav said no one was there to stop the "hooliganism" and he was not allowed to speak to the media.
He alleged that police personnel were standing but were not doing anything, saying "if the police is afraid, they can take out their uniform".
Members of JNU students' union and the Akhil Bharatiya Vidyarthi Parishad (ABVP) clashed on the campus in the evening, sources said, adding it happened during a public meeting organised by the university's teachers' association.
The students' union claimed that its president Aishe Ghosh and many other students were injured in stone-pelting by ABVP members.
But the RSS-backed students' organisation alleged its members were brutally attacked by Left-affiliated student outfits and 25 of them were injured.