The suspect stalked the victim and then punched him in the face on Seventh Avenue near West 49th Street at about 2:30 p.m., the NYPD and sources said.
After the unprovoked assault, the creep continued to trail the man up the block — before nearby officers intervened, according to sources.
Footage from the scene showed at least four cops struggling to subdue the suspect as witnesses stared in shock.
One officer had upholstered her Taser, though it was unclear if it was used.
The suspect was eventually handcuffed and hauled to a nearby hospital for a psychiatric evaluation, sources said.