The protesters caught the youth, who was passing by, and beat him up, police sources said.
ADCP (Punjab Police) Parampal Singh, said police have the video recording of the entire incident and the accused were being identified and one of them has been arrested.
Police are conducting raids at possible hideouts of rest of the accused and they would be arrested soon, he said.
Expressing concern over the incident, Jathedar of Akal Takaht Gyani Gurbachan Singh said, "no community has the right to hurt the sentiments of anyone. A baptised Sikh was inhumanly thrashed in the presence of heavy police which remained mute spectator."
He also demanded action against police officials who "were present on the spot" and witnessed the thrashing of the youth.