Bhattacharya was hit on the forehead by a microphone stand and files were also flung towards him by TMC councillors during a meeting, CPI(M) Darjeeling district committee convener Jibesh Sarkar alleged.
Trinamool Congress, however, denied that they attacked the mayor.
Left Front councillors took the mayor to hospital after the incident.
Doctors attending on him said that his injury on the forehead was mild but he was admitted to the CCU due to his falling blood pressure.
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TMC councillors threw files towards Bhattacharya and hit him with a microphone stand, Jibesh Sarkar claimed.
Leader of Opposition Ranjan Sarkar of TMC, however, said, "We did not attack the mayor. We only tore files in protest. He is just doing politics."
Later, Ranjan Sarkar and another TMC councillor Nantu Paul complained of chest pain and were admitted to hospital.
Before the commencement of the board meeting, TMC workers held a dharna in front of the Corporation building in protest against 'poor' drainage system in the city leaving to increase in mosquito menace.