Antiwar protestors, belonging to the Students Federation of India and the Socialist Unity Centre of India, threw stones and rotten eggs at the American Centre and US Consulate in Kolkata on Thursday.
They also burnt effigies of US President George W Bush and British Prime Minister Tony Blair.
They clashed with police when asked to disperse. Twenty-five people, including 2 constables, were injured in the violence, the police said.
The constables, the police said, were injured in resisting about 700 SFI activists in front of the American Centre.
SFI state secretary Apurba Chattacherjee claimed six activists, including three girls, were injured in scuffles with the police.
About 400 SUCI activists, who staged a separate demonstration targeting the nearby US Consulate, also scuffled with the police when prevented from breaking the cordon.
The SUCI claimed that 17 activists were injured in police lathi-charges.
The police, however, denied making a lathi-charge.