Shelwyn Patt, 53, was arrested aboard a B67 bus Saturday afternoon, just a few feet away from the wanted poster bearing his sketch, police said.
The toddler and and his parents were at the rally, where his father, Jose Rivera, said the violence just has to stop.
Still wanted is 20-year-old Jashid Chambers, who police belive was on the other end of the gun battle with another suspect exchanging fire for some unknown reason, authorities said.
Jose Rivera Jr., 17, was shot and killed near the entrance of Roberto Clemente Park Monday night.
Sen. Bill Perkins, a Manhattan Democrat and the first escorted out, said they are trying to get rent laws renewed or reformed. Bronx Assemblyman Jose Rivera also was arrested.