At least 10 Congress workers were injured, two seriously, in police lathi-charge when they attacked the Bharatiya Janata Party office in Lucknow.
The Congress workers were angry that BJP leaders were opposed to Congress president Sonia Gandhi becoming prime minister.
They pelted stones at the office and damaged a car, police sources said.
The police had to resort to lathi-charge to disperse the mob, which also shouted slogans against Prime Minister Atal Bihari Vajpayee and burnt effigies of BJP leaders Sushma Swaraj and Uma Bharti, the sources said.
Congress leader Virendra Madan, however, said, "The peaceful party workers were targeted by the police and 10 to 12 of them suffered injuries."
Meanwhile, several Congress members continued their hunger strike at the Uttar Pradesh Congress Committee headquarters. They sent a memorandum to the Congress chief, urging her to reconsider her decision not to be PM.
Party leader Siraj Mehndi resigned from the membership of the Vidhan Parishad, Madan said.