Around midnight, 20 NSUI activists blackened the bust of Veer Savarkar and offered a shoe chain to it.
The Congress-affiliated NSUI on Thursday said it blackened the bust of Veer Savarkar while garlanding the busts of Bhagat Singh and Netaji Subhas Chandra Bose installed by the ABVP-led Delhi University Students' Union at the varsity.
The RSS-affiliated Akhil Bharatiya Vidyarthi Parishad-led Delhi University Students' Union had installed the busts on Tuesday outside the Arts faculty.
The National Students' Union of India and the Left-backed All India Students' Association had criticised the move, saying Savarkar cannot be kept on the same pedestal as that of Subhas Chandra Bose and Bhagat Singh.
Around midnight, 20 NSUI activists blackened the bust of Savarkar and offered a shoe chain to it, the NSUI said.
They also raised slogans saying long live Bhagat Singh and Subhas Chandra Bose.
NSUI Delhi wing president Akshay Lakra said, "How could they have put the bust of Savarkar along side Subhas Chandra Bose and Bhagat Singh, that too overnight? We had to take matter in our hands. The university administration has been silent about the issue. The varsity is working at the behest of ABVP, the child unit of RSS."
The ABVP called it a "heinous act".
"Last night, the humiliation done by the NSUI to the statue of Veer Savarkar at Delhi University is a heinous act and such an act for trivial political interests reflects the Congress' thinking of the great freedom fighter of India," said Monika Chaudhary, national media convenor, ABVP.
The ABVP will make students aware about this "negative mindset" of the NSUI and its parent organisation, she said.
"In the context of this deplorable act, the ABVP urges the administration to take stern action against those involved in insulting the statue of freedom fighter," Ms Chaudhary said.
Sources said DU Proctor Neeta Sehgal issued a show cause notice to former DUSU president Shakti Singh on Wednesday demanding an answer on his act of installing the bust without due permission from the authorities within 24 hours.
Shakti Singh's tenure as DUSU president ended on August 21. The Delhi University Students' Union polls will be held on September 12.
On Wednesday, the ABVP had said it has instructed its office-bearers in the DUSU to shift the busts to the office till the time the university administration grants permission.