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When Ja’Mal Green, a prominent black activist and Bernie Sanders supporter in Chicago, saw that Donald Trump was coming to the University of Illinois-Chicago, he knew what he had to do.

“Everyone, get your tickets to this. We’re all going in!!!! ‪#‎SHUTITDOWN‬,” he posted on Facebook last week.

Little did he know, they actually would shut it down.

On Friday night, hundreds of protesters invaded Trump’s rally while thousands more marched outside, leading the GOP presidential candidate to abruptly cancel the event due to safety concerns. The night spun out from there, as angry Trump fans clashed with protesters, who saw the shutdown as a victory.

Related: Trump Blames 'Thugs' for Violence at Abandoned Chicago Rally

Protesters interrupt virtually every Trump speech. But what made Chicago different were its scale and the organization behind the effort. Hundreds of young people, mostly minorities, poured in from across the city, taking over whole sections of the arena and bracing for trouble.

And as the repeated chants of “Ber-nie” demonstrated, it was largely organized by supporters of Sanders, the Democratic presidential candidate who has struggled to win over black voters but whose revolutionary streak has excited radicals of all racial demographics.

“Remember the #TrumpRally wasn’t just luck. It took organizers from dozens of organizations and thousands of people to pull off. Great work,” tweeted People for Bernie, a large unofficial pro-Sanders organization founded by veterans of the Occupy movement and other leftist activists.

Remember the #TrumpRally wasn't just luck. It took organizers from dozens of organizations and thousands of people to pull off. Great work. — People For Bernie (@People4Bernie) March 12, 2016

Sanders’ campaign was not involved with the protest.

"As is the case virtually every day, Donald Trump is showing the American people that he is a pathological liar. Obviously, while I appreciate that we had supporters at Trump's rally in Chicago, our campaign did not organize the protests," Sanders said in a statement Saturday.

Still, some felt encouraged by the showing. If the candidate doesn’t win the Democratic presidential nomination, the anti-Trump showing in Chicago foreshadows a possible future avenue for his movement in the general election, in which Trump is the most likely Republican nominee.

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Friday night happened, according to various activists involved, thanks to a confluence of events.

The pump was primed in Chicago, thanks to now near-constant demonstrations against Mayor Rahm Emanuel over the shooting of Laquan McDonald and other issues. Trump chose a venue, the University of Illinois-Chicago campus, in the heart of the city, where student organizers whipped a demonstration together. And activists tapped into existing networks of pro-Bernie Sanders and Black Lives Matter activists.

Rumors about Trump’s visit started flying around the UIC campus Friday evening. Students were not happy.

On Monday night, there was a meeting. The lecture hall, meant to hold 100 students, overflowed with representatives of student groups from the Black Student Union to Fearless and Undocumented. They decided they would march outside the rally to protest.

Photos: Protesters and Trump Supporters Clash in Chicago

“He’s marginalized and dehumanized a lot of different groups, and they all come together,” said Juan Rojas, one of the student organizers.

A Facebook page was started to promote the protest. By the night of the Trump rally, more than 11,600 people had RSVP’d on the page saying they would attend the event. Another 19,000 said they were “interested.” Organizers were shocked when Facebook’s analytics said the page reached more than 1.5 million users.

The page explained how to acquire tickets to the Trump rally, complete with links, instructions on where and when to meet, and exhortations to remain peaceful.

Jorge Mena, an undocumented graduate student at UIC, started a petition on MoveOn.org calling on the school’s administration to cancel the event. The petition garnered more than 50,000 signatures, and once brass at MoveOn, which has endorsed Sanders, caught wind of the UIC backlash, they wanted to help.

MoveOn chipped in money to get signs and a banner printed and blasted out an email to members in the Chicago area encouraging them to join the protest.

With just four days to plan, organizers said more than 1,000 students turned out for the march, along with thousands of community members.

Meanwhile, on the night of the Trump rally, organizers who planned to disrupt the event from inside had designated multiple rallying points around the venue to avoid arousing suspicion of authorities with large congregations. Some met at the Papa John’s, others at the Quad.

As activists slipped into the lines, they were told to blend in with the crowd and act natural.

Inside, about 100 protesters received coveted orange wristbands allowing them access to the floor. Even as organizers tried to maintain calm, some scuffles with Trump fans started right away, and police began removing people. Secret Service agents recognized Green and pulled him outside, he said, but he quickly changed into a friend’s hoodie and slid back in.

The plan was to wait until Trump took the stage, then wait for the applause to die down and have all the protesters erupt at once. But they never made it that far. Instead of Trump, an official announced the event was canceled.

Related: Rubio: If Trump's Our Nominee, GOP Will Be 'Defined By' Violence

At that point, “we just went crazy,” said Green.

Jedidiah Brown, another organizer, rushed the stage to rip up a Trump sign before being removed. He was put in a headlock by a Trump supporter and had a water bottle thrown at his head.

“It became a Bernie Sanders rally,” he said. Hundreds of supporters chanted Sanders’ name, and some carried signs for the senator. Brown guessed the vast majority of the diverse protesters were Sanders supporters.

Matthew Ross, who created several event pages promoting the protest, said the activism around the McDonald shooting played directly into the event. “That kind of just carried over to this,” he said.

“We’ll just keep a lookout and see if [Trump] tries to come back to Chicago,” Ross said. “Hopefully, there’s a domino effect.”