Republican presidential front-runner Donald Trump abruptly canceled his Friday night rally at the University of Illinois at Chicago Pavilion, citing security concerns as thousands of protesters gathered outside.

“Rather than having everybody get in and mix it up, I thought it would be a wise thing, after speaking with law enforcement, a wise thing to postpone the rally,” Trump told MSNBC.

Trump said he “felt that it was just safer” to cancel. “I don't want to see anybody hurt.”

At a Republican fundraising event in a northwest Chicago suburb, a rival for the GOP nomination, Texas Sen. Ted Cruz, denounced any violence but said that “any candidate is responsible for the culture of the campaign.”

“And when you have a campaign that disrespects the voters, when you have a campaign that affirmatively encourages violence, when you have a campaign that is facing allegations of physical violence against members of the press, you create an environment that only encourages this sort of nasty discourse,” Cruz said.

The Chicago cancellation came after concerns about violence at or outside Trump speeches built for months, in part a reflection of the candidate's own heated comments toward protesters who interrupt his events, saying he would like to “punch” them or see them “roughed up.”

But just as Trump's supporters are passionate, passionate opposition exists toward him — spawned in part by controversial statements and pledges, such as walling off the U.S. border with Mexico and temporary halt Muslim immigration.

Beofre his arrival in Chicago on Friday, Trump appeared at a rally in St. Louis where he mocked concerns over violent acts against protesters.

“They're allowed to get up and interrupt us horribly and we have to be very, very gentle,” Trump said in response to one of nearly a dozen interruptions as he spoke at the Peabody Opera House.

“They can swing and hit people, but if we hit them back, it's a terrible, terrible thing, right?”

Earlier this week, a Trump supporter was charged with assault after multiple videos showed him sucker-punching a protester who was being escorted out of a campaign rally Wednesday night in Fayetteville, N.C.

In addition, Michelle Fields, a reporter for the conservative news Internet site Breitbart, filed a police report in Jupiter, Fla., alleging she was assaulted by a person identified by a Washington Post reporter as Trump's campaign manager, Corey Lewandowski.

Fields said someone “grabbed me tightly by the arm and yanked me down” after a Tuesday news conference. The Trump campaign denied that such an act ever took place.

In Chicago, the Trump campaign pulled the plug on the rally about a half-hour after it was supposed to start.

Protesters inside the arena cheered and danced.

There were chants of “We stopped Trump.”

Trump supporters chanted “We want Trump” before the crowd began filing out, joining thousands of protesters outside. There were some clashes and pushing, images captured by cable news networks.

Some Trump backers were forced to move through a gantlet of protesters, many of whom were shouting, calling them “bigots” and singing, “Sha-na-na-na, na-na-na-na, hey, hey, goodbye.”

The event's cancellation was a culmination of Trump's rhetoric colliding with opposition from within a diverse and Democratic-oriented big-city environment where protests are not unfamiliar, at times with disparate groups banding together.

Almost since Trump announced the Chicago rally, groups had been mobilizing, securing free tickets to get into the UIC Pavilion either to attend and protest, or to prevent Trump supporters from gaining access to the 10,000 seat arena.

UIC faculty and staff signed a letter asking administrators to cancel the rally because it could turn violent.

And more than 40,000 signatures were collected on a petition started by a student leader asking how security would be handled and who would pay for it.

Long before the Trump rally's scheduled start, hundreds of young people, many of whom appeared to be students, filled sections toward the back of the arena.

At about 5:30 p.m. when two protesters were removed by security personnel, the size of the protest contingent became clear when they loudly chanted, “Let them stay!” Trump's supporters responded by chanting, “USA! USA!”

The contingent of protesters continued to get louder, some tearing Trump banners in half as a group of police officers began to remove some of them, one by one.

Outside the arena, thousands of protesters gathered after a march from the university's quad. A phalanx of Chicago police officers separated protesters from Trump supporters across the street.

Protesters chanted “Hey, hey. Ho, ho. Donald Trump has got to go.”

A few Trump backers lashed back at demonstrators, shouting, “Build the wall!” a reference to Trump's pledge to build a wall on the Mexican border.

Aimee Bass, a 49-year-old music teacher from Chicago, said she came out to voice her opposition to Trump, because “he's so unqualified to run for president.”

Nineteen-year-old Radia Mchabcheb said she came from a Chicago suburb to stand against Trump. “There's no support for hateful kinds of speech, especially for presidential candidates,” she said.

Diane Szafranski, a 48-year-old homemaker from Cary, Ill., brought her daughter, Caitlin, 10, to see Trump. She said she had no problem with Trump's past controversial statements. “He's not politically correct,” Szafranski said. “He's not taking any crap from anybody, which I love.

“I love him, he's awesome,” Szafranski's daughter said. “He wants to get rid of ISIS, he wants to build a wall to help our country.”

Farice Campbell, a 21-year-old African-American man from Chicago, said, “We came to see in real life how this all plays out.”

The Associated Press contributed to this article.