Amid growing security concerns, Republican presidential candidate Donald Trump's campaign canceled a Chicago rally on March 11. (Victoria Walker/The Washington Post)

The push to disrupt Donald Trump’s campaign rally began a week ago, when news first broke that the Republican presidential candidate would appear at the University of Illinois campus here on this city’s West Side.

Student leaders of campus organizations such as the Black Student Union and Muslim Student Association began organizing their own rally and march to the rally venue; a Facebook page publicizing the efforts attracted 11,000 people.

Activist organizations, also largely comprised of young people, were making their own plans, rooted in the police-shootings demonstrations that have rocked this city for months. Chicago high-school student and frequent demonstrator Cameron Miller, 18, said he and others met twice last week at a local Dunkin’ Donuts to examine a floor plan of the pavilion, with the idea of storming the stage in unison during Trump’s speech. He said they were prepared to endure physical violence for their actions but agreed not to fight back.

By late Friday, thousands were camped outside the pavilion where Trump was scheduled to give his speech, with thousands more from both sides inside the building. Scattered jeerings and skirmishes broke out, and police began removing some of the most disruptive.

Then, as the tensions were rising, an announcer told the crowd that Trump had arrived in Chicago — but that after consulting with security, he decided to postpone the event.

Republican presidential contender Donald Trump canceled a campaign rally in Chicago on March 11 amid security concerns, after thousands of people protested the rally inside and outside the University of Illinois arena. (Victoria Walker/The Washington Post)

“We stopped Trump!” the protesters chanted, pumping their fists into the air. “We stopped Trump! We stopped Trump!”

The success in shutting down Trump’s appearance was both organized and organic — the latest clash in a city where an energized activist network has frequently disrupted daily life with protests over high-profile police shootings of black men. In November, Black Friday shopping ended when thousands of protestors lined Michigan Avenue, refusing entry to shoppers.

The University of Illinois at Chicago is one of the most diverse college campuses in the country, with no racial or ethnic majority and a student body that is nearly 30 percent Latino. Many students here are particularly outraged by Trump’s proposals to deport 11 million illegal immigrants, bar foreign Muslims from entering the country and build a giant wall on the U.S.-Mexico border.

“Trump is putting out a lot of hatred, saying that marginalized groups shouldn’t be welcomed here at UIC or Chicago,” said Juan Rojas, a 19-year-old sophomore.

With the Illinois GOP primary on Tuesday, Trump could have held a rally in one of the more conservative suburbs or in a deep-red rural town. Instead, he opted for a university arena in the heart of deeply blue Chicago, guaranteeing that he would have protesters and heavy media coverage. Trump has selected settings like this before, turning out massive — and mostly white — crowds in Democrat-leaning towns, revealing the pent-up anger and frustrations of conservatives who often do not identify with either political party.

On Saturday morning, Trump tweeted: “The organized group of people, many of them thugs, who shut down our First Amendment rights in Chicago, have totally energized America!”

Dick Simpson, a UIC political science professor, said the Friday rally reminded him of the 1968 Democratic National Convention in Chicago, where protestors famously disrupted proceedings for days.

“Then, people were mad about segregation and the war in Vietnam and, while the issues are somewhat different, this time it’s the same kind of anger,” said Simpson, who was then the Illinois campaign manager for candidate Sen. Eugene McCarthy.

“There will be copycat rallies, because the protestors will have seen this as successful in blocking Trump,” he added. “Emotions are so strong that in any city with a large minority population, it will become pretty tempting.”

One of the organizers of the November Black Friday protests, 17-year-old Lamon Reccord, said the Trump rally provided an opportunity to show the city that the police reform movement was still active. Days before the rally, he used his extensive list of emails and phone numbers to energize followers to show up outside the pavilion.

“We need more people to make the public aware of all these political issues we care about,” he said.

Andrea Varias, 36, a 2003 UIC graduate, marched with former professors and colleagues Friday afternoon. “Having [Trump’s] presence here, especially on such an awesome campus as UIC, which prides itself as being diverse, we felt we needed to convey the message that we are not down for his hate speech,” Varias said.

In the end, few arrests were made and, unlike some recent Trump rallies, the conflict was mostly limited to name-calling and tussles. A statement from Trump’s campaign suggested that the rally was canceled in consultation with law enforcement officials; the Chicago Police Department and other agencies said the campaign canceled the event on its own.

Many Trump supporters were livid at the outcome.

“I think we’re too divided to become united at this point, and it may someday lead to real revolution, the way things are going,” said Mark Falkingham, 52, an engineer from the suburbs who immigrated from Canada and plans to vote for Trump on Tuesday. “I’m really upset that he postponed, and I’m embarrassed that this school is in the city of Chicago and that the students are acting like this tonight. I think it’s disgraceful, personally.”

About two hours after the cancellation, the streets near the arena were starting to clear up. Suzanne Monk, a 44-year-old music shop owner, continued to stand watch holding a large blue Trump flag.

“It is important that if the other side believes they can quash our free speech, that we don’t let them win,” said Monk, who arrived at the rally 3 1/2 hours early. “So, I’m going to stand out here until they actually make us all go home. . . . We’re just trying to hear our candidate speak and engage in the political process peacefully.”