The man was later identified as Debashish Acharya, a resident of Tamluk in East Midnapore.
Mr Banerjee was slapped while he was on stage at the meeting that was being held in Chandipur in East Midnapore district.
Mr Acharya was then beaten up by Trinamool workers and has been admitted to a local hospital.
The party has claimed that Mr Acharya was not a member of the Trinamool Congress but an "outsider".
Mr Acharya was apparently allowed to go on stage after he told party workers that he wanted to take a photo of Mr Banerjee on his phone. It is not yet clear why he attacked Mr Banerjee.
There is some speculation that the incident could have been the result of internal party problems. Some have speculated that the attacker may be a supporter of Trinamool leader Suvendu Adhikari, the party MP from Midnapore's Tamluk.
There is resentment reportedly at the grassroots about Mr Adhikari being replaced as head of the Trinamool Youth Congress some months ago by Soumitra Khan, and more recently by Abhishek Banerjee.
Mr Adhikari has denied any involvement in the incident.
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.