thanksgiving day event, macy's thanksgiving day parade. don dahler was there. ♪ ♪ >> reporter: the largest contingent of heavily armed police officers in the parade's history stood along the parade's two-and-a-half-mile route. they were protecting the 3.5-million onlookers who crowded streets, 20 teen deepin some places. some were concerned about terrorism, some not. >> we're super excited. the weather turned out perfect so we couldn't ask for anything better. >> reporter: in their midst, undercover cops moved among them and sniper teams scanned from rooftops. in downtown manhattan, dozens of officers also kept an eye on things in a massive room named the joint operations center. chief james o'neill gave us a tour. can't help but notice all these gigantic screens. what kind of information would you have up there? >> we have cameras all around the city that we can choose. if there's an event somewhere in the the city we can go to that neighborhood. >> reporter: during major events like the parades the center is staffed with representatives of various state