The Chief Minister had gone to Panipat, some 80-km from Delhi, to inaugurate and lay foundation stone of various projects.
As Hooda's Maruti Gypsy was making its way through the crowded places, a youth identified as Kamal Mukhija broke the security ring and slapped the Chief Minister.
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A senior police officer told PTI that the youth, who was suffering from depression, was immediately detained.
A case had been registered against him under various sections of the IPC, the officer said.