Some 500 activists stormed into the Congress office in Lucknow, attacked people and damaged cars.
The activists are believed to be from the Bundelkhand Adhikar Sena, floated by a man called Guddu Rajput, who was reportedly rebuffed in his bid to contest as a Congress candidate from Hamirpur. He is said to be the son of a former MP, Gangacharan Rajput.
The attack went on for an hour before the police stepped in. The Congress says 40 of its workers were injured.