The incident happened earlier in the day when Hardik was addressing a gathering at the Jan Akrosh Sabha.
Hardik was in the middle of his speech when the attacker got on to the stage and slapped him.
Subsequently, a scuffle broke out between the supporters of Hardik and the attacker.
This incident comes a day after a shoe was hurled at BJP's national spokesperson GVL Narasimha Rao while he was addressing a media briefing in New Delhi. He, however, had a narrow escape as the shoe did not hit him.
Prior to joining Congress, Hardik had said he would help strengthen the ideology of the party and take it to the villages.
Gujarat will go to polls on April 23 for the 26 Lok Sabha seats at stake. Counting of votes will take place on May 23.