Aerial footage showed fights breaking out, rioters hurling stones and chairs at police officers, climbing on to a subway station entrance and tearing down metal barriers.
NYPD chief of department Jeffrey Maddrey said at a press conference that the teenagers broke into a construction site in the square and threw rocks, bottles, paint cans and lit fireworks at officers.
“Our officers were attacked, we were crushed, we were pushed, I had officers on the floor,” Mr Maddrey said.
“You had people were walking around with shovels, axes... a lot of young people got hurt.”
The crowd of mostly teenagers crammed into Union Square Park after a social media post promoting the 4pm giveaway went viral. At around 3pm, an hour before an anticipated appearance by Mr Cenat, violence suddenly exploded.
The NYPD put out a citywide alert for every available officer across the five boroughs — up to 1,000 officers — to go straight to Union Square to deal with the rioting.
A line of police with clubs and shields smashed through the crowd at around 4.30pm, and began to regain control of the situation, according to reporters on the ground.
Mr Cenat, 21, a YouTube star and live-streamer who has 6.5 million followers on Twitch, was surrounded by hundreds of fans in Union Square as the chaos unfolded. He left the square with a police escort and has since been taken in for questioning, Mr Maddrey said.
Police are considering prosecuting him for incitement to riot and “other charges”, he added.
Mr Maddrey confirmed that there had been no coordination with police ahead of the event.
A large number of arrests had been made, and police were tallying exact numbers.
One police officer was reportedly jumped by rioters and severely beaten. Mr Maddrey said he had been in the thick of the riot, and suffered injuries after being hit with flying objects.
“As you see I’m a little embarrassed, I’m very dirty right here.
“I was in the middle of the crowd, I was hit with a couple of objects as well. I think I have a couple of injuries but I’ll figure that out when I have time to think about it.
“I had officers who were hit and were hurt, but they stayed to help us resolve this.”
Mr Maddrey saw young people in the crowd with severe injuries, and suffering panic attacks and anxiety attacks.
“People were suffering out here. It was a lot of people, it was uncontrolled, it took us a while to get it under control, and a lot of young people got hurt.”
Many of the injuries were sustained by teenagers throwing objects at one another, he added.
He said police were still gauging the exact number of injuries.
Earlier, CBS News New York showed live video footage showing several people had climbed onto a roof at the entrance to Union Square.
“Our officers used great restraint despite the aggression they were met with,” Mr Maddrey said.
The gathering was declared an unlawful assembly, and officers tried to encourage them to leave the area.
“After numerous warnings and assailed, we started to make arrests and clear out the park.”
Traffic was snarled in streets surrounding the square, with groups of youths climbing on top of vehicles.
People could be seen pounding on the side of a bus, and climbing on top of another bus and jumping up and down.
The MTA prevented subway trains from stopping at Union Square for about an hour, before resuming regular services just before 5pm.
A large amount of paint was splattered across the south side of Union Square Park.
Police erected metal barricades to try to push the crowd back.
They issued a warning over loudspeakers: “This gathering is unlawful. You are ordered to disperse.
“If you leave voluntarily, no charges will be placed against you. If you refuse to leave you will be arrested for unlawful assembly.”
As of 6pm, police were monitoring groups of rioters who had spread out around Manhattan. Mr Maddrey said.
Officers were still encountering resistance and making arrests, Mr Maddrey.
“We have encountered things like this before, but never to this level of dangerousness where young people would not listen to our commands, they were fighting each other and they were hurting each other.”
“This is the power of social media, the danger of social media,” Mr Maddrey said. “We can’t allow this to happen again.”
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