The youth, wearing a cap, mounted the stage from which Banerjee, an MP, was addressing the gathering and while clicking photos on his cell phone suddenly lunged towards Banerjee and slapped him, taking him off-guard, police officials said.
TMC workers present on the stage caught the youth and beat him up.
TMC secretary-general Partha Chattejee said he had asked the district SP to inquire into the incident and submit a report immediately.
"I have asked the SP to conduct an inquiry and submit a report to the government. I also talked to Abhishek over phone. He told me that he had been slapped, received injury on his left eye and physically assaulted, but he never stopped addressing the gathering," Chatterjee said.
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TMC leader and Rajya Sabha member Derek O' Brien said, "On the pretext of taking a picture he (Abhishek) was slapped. We are in the hurly-burly of politics. We will look into this and we will find out whether there is any outsider involved."
The BJP condemned the incident but described it as a result of the TMC's internal feud.
"We do not support such incidents. We condemn the incident because we believe there cannot be any violence in politics," party's national secretary Siddharth Nath Singh told newspersons.