The West Bengal unit of the BJP on Thursday accused the police of forcibly entering its headquarters and "mercilessly beating up" the workers on a day when violence erupted here during a protest march organised by the party.

Terming the incident as "shameful", West Bengal Bharatiya Janata Party Secretary Raju Banerjee charged the police with acting as cadres of the state's ruling Trinamool Congress.

"It is a shameful day in the history of West Bengal's politics. Police have forcefully entered our party office and mercilessly beaten up the party workers present there," Mukherjee said.

"Today's incident clearly proves that the state police actually is an appendage of the Trinamool Congress. It seems there is a competition between the police and the Trinamool backed hooligans to show who are more faithful cadres of the ruling party," he alleged.

The BJP leader also alleged police attacked their party activists without any provocation and used abusive language.

"Police suddenly attacked us without any provocation. The party office was empty at that point as everyone was busy with the rallies. Only a few injured people were being treated inside. We have the video footage of how they attacked us and what kind of abusive language they used," he said.

Kolkata resembled a war zone for much of Thursday afternoon as the police and BJP supporters fought pitched battles on various city streets. 

While the activists hurled stones and tried to break barricades in a desperate attempt to reach the Kolkata police headquarters, the security forces used water canons, batons and tear gas shells to drive away the rallyists.