An AAP councillor who was with Mr Kumar has said she was also threatened.
In a shocking incident, Kanhaiya Kumar, the Congress' Lok Sabha candidate from North East Delhi, has been assaulted by seven to eight people and had black ink thrown at him while campaigning in the national capital.
Videos of the assault, which took place in East Delhi's New Usmanpur area on Friday, have now gone viral. Two of the alleged attackers have also released videos boasting that they had thrashed the Congress leader and said they did so because he had raised slogans on breaking up the country and had spoken against the Indian Army.
In a complaint, Chhaya Gaurav Sharma, the AAP councillor from Brahmapuri, said she and Mr Kumar were leaving the party's office in Kartar Nagar when seven to eight people garlanded Mr Kumar, threw ink on him and then thrashed him. She said three to four women were also injured, and a woman journalist fell into a drain.
Ms Sharma alleged the attackers also dragged her to a corner using her stole and threatened to kill her and her husband.
Delhi Police officials confirmed that they have received Ms Sharma's complaint. They said they are verifying the videos and investigation has begun.
In videos released after the incident, two of the alleged attackers - at least one of whom can be seen holding a garland in the footage of the assault - showed their reddened hands and said they did what they did because Mr Kanhaiya had raised slogans saying the country would be broken into parts. This was a reference to the alleged slogans raised at the Jawaharlal Nehru University (JNU) in 2016, when Mr Kumar was the JNU Students' Union president.
"Hamne chaante ka jawaab diya hai (we have given an answer with our attack)," the two men said. Calling themselves 'Sanatani lions', they claimed Mr Kumar had also insulted the Indian Army and they had "treated" him.
The 37-year-old Congress leader is contesting against the BJP's Manoj Tiwari, who is the sitting Member of Parliament from the constituency and also the lone Delhi MP who was not replaced by the party. Delhi will vote in the sixth phase on May 25.