Republican presidential front-runner Donald Trump abruptly canceled his rally at the University of Illinois on Friday as skirmishes erupted among thousands of protesters gathered inside and outside the arena.
Police descended to help clear the arena, even as further chaos erupted in the streets outside.
Trump said the decision was made to cancel the event after consulting with authorities about the possibility of problems.
"They said it would be better not to do it," Trump told CNN after the cancellation. "People could get hurt. I didn't want people to get hurt."
Chicago police officials said they did not order the event shut down and were not consulted before the Trump campaign decided to cancel it.
"We were confident we had the proper amount of resources dedicated to the event," Interim Supt. of Police John Escalante said at a news conference Friday night.
Trump's expression of concern was a change in tone from an earlier rally Friday in St. Louis, when he continued to taunt those who interrupt his events while promising that police and security would be "gentle" as they removed them.
"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 in St. Louis at the regal Peabody Opera House. "They can swing and hit people, but if we hit them back it's a terrible, terrible thing, right?"
In recent weeks, Trump has been accused of urging his crowd to harm protesters, and there have been increasing reports of violence against protesters and the media at his events.
More controversy emerged over a video showing an African American protestor apparently being sucker-punched by a Trump supporter.
President Obama was critical of the Republican front-runner shortly before the cancelled rally, zeroing in on Trump's comments about Muslims and Latinos and blaming the GOP establishment for not reacting to them more critically.
"This is the guy, remember, who was sure that I was born in Kenya. Who just wouldn't let it go," said Obama, who released his long-form birth certificate in 2011 as Trump increasingly hammered the false notion that the president was born overseas and therefore ineligible to be head of state.
"And all this same Republican establishment, they weren't saying nothing. As long as it was directed at me, they were fine with it," Obama said at a Democratic fundraiser in Austin. "They thought it was a hoot. Wanted to get his endorsement. And then now, suddenly, we're shocked that there's gambling going on in this establishment."
Fellow GOP candidates were quick to condemn Friday's events.
360 degree panorama of Trump rally at UIC
Texas Sen. Ted Cruz said Trump was to blame for the clashes. "Any candidate is responsible for the culture of the campaign," he said at a fundraising event in suburban Chicago.
"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.
Florida Sen. Marco Rubio said he did not blame Trump but said there is "real significant anger and frustration at the direction of the country."
Obama, he said, shared part of the responsibility.
"President Obama has spent the last eight years dividing Americans along have and have-nots, along ethnic lines, racial lines in order to win elections," Rubio told Fox News. "I think this has gone to the next level here, and I think we're seeing the consequences of it."
Asked if his campaign rhetoric had encouraged such clashes, Trump said most of the problems at past rallies had been instigated by protesters, not his supporters.
"We will have protesters stand up and be very, very abusive, unbelievably abusive, and in some cases swinging, punching and swinging," he said. "Overall, I think we've been very mild with protestors."
His rallies have attracted tens of thousands of supporters, he said. "I mean, it's a love fest in the rallies themselves," he said. "There's great love in those big stadiums."
While Trump has faced interruptions during his speeches for months, he had not been confronted with the type of large, organized protest that unfolded Friday in Chicago.
Hours before the scheduled start of the event, 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, "U.S.A.! U.S.A.!"
The contingent of protesters continued to get louder, some tearing Trump banners in half as a group of five police officers began to remove some of the demonstrators one-by-one. Police appeared to handcuff one man before taking him out of the arena.
At one point, a protester shouted an expletive with Trump's name while holding a "No Hate" sign. People in the upper balcony threw debris at her.
The cancellation was announced more than 30 minutes after the event was scheduled to begin, when a voice came over the sound system informing the thousands in attendance that the event had been postponed due to "security concerns."
Hundreds of anti-Trump demonstrators erupted in celebration while the GOP front-runner's supporters stood in stunned silence. Many of them quietly held signs that read "The silent majority is for Trump" while protesters wildly chanted "we stopped Trump!"
Officials moved to empty the arena with an announcement of "please exit the building" over the public-address system. Thousands of Trump backers and protesters filed out, joining the thousands of protesters outside. There were some clashes, captured by cable news networks.
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.
At one point, protesters briefly blocked the Eisenhower Expressway. Police dispersed them, but later protesters began blocking the expressway's westbound ramp.
Eventually, police managed to clear the four-story parking deck of protesters and drivers. On the street outside, hundreds of people still milled about. UIC police carrying plastic handcuffs were assisting Chicago police.
Some Trump backers were forced to move through a gantlet of protesters, many of whom were shouting at them, calling them "bigots."
One group of supporters and demonstrators nearly exchanged blows. Amid the chaotic scene, some demonstrators peddled T-shirts with Trump's name and an expletive. One little girl sported an anti-Trump sign with a pun: "We shall overcomb."
As the crowds dispersed, hundreds of protesters stood in the median along Harrison Street, some chanting "Muslim lives matter!"
They held up signs, including a few that depicted Trump in the white sheets of the Ku Klux Klan. Those signs had scrawled on them "Mein Trumpf" — a play on comparisons some have made to Adolf Hitler.
More than a dozen Chicago police officers on bicycles tried to block the crowd from occupying Harrison Street.
A large cluster of protesters also moved toward a nearby parking deck, blocking numerous cars from leaving at the Harrison exit.
Protesters started booing and jeering motorists as they started to exit the parking garage. "Go back to the suburbs!" one protester shouted.
More than a dozen police officers on horseback formed a line to prevent some of the protesters from disrupting the traffic as the cars drove away.
A helicopter buzzed overhead.
"Where's Donald Trump?" one protester shouted angrily.
The GOP leader of an anti-Donald Trump effort said the violence was unsurprising given the statements Trump has made.
"When you use incendiary speech and you encourage your own supporters and your staff to be violent and aggressive, and when you ignore the cries of victims in your midst, it shouldn't come as a surprise that violence and aggression follow you," said Katie Packer, head of the Our Principles PAC, and former deputy campaign manager to 2012 GOP nominee Mitt Romney.
The group is spending millions advertising heavily against Trump in the run-up to Tuesday's primary vote, notably in Florida. They have received increased funding and attention in recent days as establishment Republicans try to unite and stop Trump from winning the GOP nomination.
Packer said that it was wrong for protests to shut down a political rally, but she also said it was inevitable given Trump's rhetoric.
"It's only a matter of time before someone gets seriously hurt. And when that happens, Donald Trump will have a lot to answer for," she said.
The protest against Trump represented numerous disparate groups in a city where protesting has a long tradition.
Almost since Trump announced the Chicago rally a week ago, groups were mobilizing.
On Monday, a group of Latino elected officials led by Democratic U.S. Rep. Luis Gutierrez, of Chicago, called on people to show up and express opposition to Trump's candidacy.
In addition, UIC faculty and staff had signed off on 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.
A 24-hour vigil leading up to Trump's campaign stop and organized by Latino community leaders began late Thursday.
Among the thousands who had gathered to see Trump, some said they appreciated the candidate's forthright statements.
"He's not politically correct," said Diane Szafranski, a 48-year-old homemaker who brought her 10-year-old daughter to see Trump. "He's not taking any crap from anybody, which I love."
"He's self-funding," Szafranski added. "He's not relying on the lobbyists. He's not going to owe anybody."
One Trump backer, Jeff Black, handed out anti-Hillary Clinton buttons reading "Hillary for prison."
But Aimee Bass, a 49-year-old music teacher, said she came out to voice her opposition to Trump.
He had every right to be a businessman, she said, but "he's so unqualified to run for president."
Tribune reporters Rick Pearson, Annie Sweeney, Jeremy Gorner and Matt McCall and Los Angeles Times reporters Seema Mehta and Melanie Mason, along with the Associated Press, contributed to this report.