Delhi police are investigating how masked men burst into a premier university and attacked a student protest with sticks and rods, an officer said on Monday, the latest incident to ignite criticism of India's ruling Hindu nationalists.

The attack took place at a university long seen as a bastion of left-wing politics, and came as students nationwide lead a campaign against a citizenship law introduced last month by Prime Minister Narendra Modi that is seen as discriminating against Muslims.

Some have blamed the incident on a student union tied to Modi's Bharatiya Janata Party - though it has denied involvement.