CHICAGO — Donald Trump hastily postponed his Friday night rally in Chicago because of “growing safety concerns” created by thousands of protesters inside and outside of an arena at the University of Illinois. The decision immediately sparked nasty verbal and physical fights between protesters and Trump supporters who had been eager to see him that night.

The Republican front-runner’s rallies have become increasingly violent in the past two weeks, and Trump’s remarks are often interrupted by protesters denouncing his controversial stances, especially those on immigration and the treatment of Muslims. But Trump has never had to cancel a rally because of the threat of protesters.

A crowd of more than 9,000 learned of the cancellation about 6:35 p.m. Central time, more than half an hour after Trump was scheduled to take the stage. The thousands of protesters immediately burst into cheers and began chanting: "We stopped Trump! We stopped Trump!" Many of Trump's supporters, who had waited hours to see him, seemed stunned and a few tried a chant of their own, without much luck: "USA! USA!" As the two sides reacted to the news, skirmishes broke out in the crowd and spilled out of the arena into a mass protest outside.

"It sent a message that Chicago is a very liberal city and it will always be a liberal city because it does not promote hate — it promotes love and it promotes prosperity," said Farris Ahmad, 23, a protester and junior political science major at the College of DuPage. He was cut off by a group of police sparring with a Trump supporter who had ripped away a cloth sign being held by a protester, sparking a profanity-filled tussle.

"I'm so hurt, I'm so upset — we were so excited. I just can't believe there's that many people that would come in here and destroy this," said Valerie Schmitt, 65, a Trump supporter from Naperville, Ill., who teared up as she watched the protesters celebrate. "It's one thing if they are outside, but this is just a shame that they did this."

The Chicago Police Department said it was informed shortly before 6:30 p.m. that the Trump campaign had canceled the event, an announcement that took the department by surprise, according to the police chief.

The department "had no role, we were not consulted or provided an opinion" about whether or not to cancel the event, interim police superintendent John J. Escalante said at a news briefing Friday night. A department spokesman said that police did not issue any public-safety threats or safety risks before the cancellation.

The University of Illinois Police Department confirmed Saturday that it was not consulted before the event's cancellation.

The Trump campaign released a statement Friday: “Mr. Trump just arrived in Chicago and after meeting with law enforcement has determined that for the safety of all of the tens of thousands of people that have gathered in and around the arena, tonight’s rally will be postponed to another date.”

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

More than an hour before Trump was set to arrive in Chicago, tension was already high in the arena at the University of Illinois at Chicago, where several thousand protesters eager to denounce his message waited alongside several thousand Trump supporters eager to hear him speak. Outside, thousands more gathered. At first the altercations were verbal, with protesters yelling at supporters and vice versa.

In an arena section dominated by protesters, a black man dramatically ripped a Trump campaign sign in half and then quietly held up the two pieces. A young Latino man yelled at a small group of Trump supporters, flashing his two middle fingers. A small group of women repeatedly shouted: "F--- Trump!" As police selectively escorted some of the most disruptive protesters out, the crowd shouted: "Let them stay!"

While most rallygoers quietly watched this unfold, a few Trump supporters directly engaged with the protesters, resulting in nasty verbal confrontations. Dozens of police officers worked to keep the crowd calm and escort out the most disruptive people from both sides. One exasperated Trump supporter walked past the protesters and shouted: "God! Why do you create fools?"

As soon as the cancellation was announced, shoving matches broke out between the two groups, and police tried to break up one scuffle after another. Everyone moved outside, and the crowd grew in numbers and the altercations continued. Hundreds of protesters gathered in the streets and in a parking garage and clashed with Trump supporters leaving the rally.

Trump called Friday night into MSNBC and said on air that he did "the right thing" by canceling his rally in Chicago.

“You can’t even have a rally in a major city in this country anymore without violence or potential violence,” Trump said. “I didn’t want to see the real violence, and that’s why I decided to call it off.”

Trump added, “You have so much anger in the country — it’s just anger in the country, and I don't think it’s directed at me or anything. It’s just directed at what’s been going on for years.”

In an interview with anchor Chris Matthews, Trump was defensive and argued that the anger boiling over at his rallies had been building for years and was not spurred by his campaign alone.

“We have a very divided country,” Trump said. “We have a country that’s so divided that maybe even you don’t understand it. I’ve never seen anything like it.”

When Matthews asked whether Trump would tell his supporters not to engage with protesters, the billionaire said he wanted them to leave the Chicago arena peacefully.

“I don't want to see people hurt or worse,” he said.

Two hundred Chicago police officers were outside the event, and 100 more were called in to the area after the department heard rumors Trump might cancel. The University of Illinois at Chicago's police department was responsible for security inside the building, Escalante said.

Kevin Booker, chief of the university police, said in a statement that "the vast majority of attendees at today’s events exercised their Constitutional rights of free speech and free assembly peacefully.”

Booker said his department had worked with the Chicago police, Secret Service and Illinois State Police, along with as well as campaign and protest organizers, in addressing security for the event.

“The abrupt announcement of the cancellation of the event created challenges in managing an orderly exit from the Pavilion, which nonetheless, was accomplished with no injuries or arrests,” Booker said.

After the event was canceled, the university police called on city police officers to help them inside after fights began to break out in the pavilion, Escalante said.

CBS News reported Friday that Sopan Deb, a journalist covering the Trump campaign, had been detained by law enforcement. CBS said in a statement Saturday morning that Deb was "thrown to the ground by law enforcement" while filming footage of a dispute between police officers and protesters. He repeatedly identified himself as a member of the news media and offered credentials. Deb was charged by the Illinois State Police with resisting arrest, according to CBS, though video footage of the incident published by CBS shows no sign of a struggle.

Several physical altercations have occurred at Donald Trump rallies in recent months. (Monica Akhtar/The Washington Post)

Former secretary of state Hillary Clinton, the front-runner for the Democratic nomination, condemned Friday night’s violence and blamed “divisive rhetoric” for stoking tensions.

“All of us, no matter what party we belong to or what views we hold, should not only say loudly and clearly that violence has no place in our politics, we should use our words and deeds to bring Americans together,” she said in a written statement.

Clinton also called for unity, pointing to a shooting last year in Charleston, S.C., when nine people were gunned down in a church.

“The families of those victims came together and melted hearts in the statehouse and the confederate flag came down,” she said. “That should be the model we strive for to overcome painful divisions in our country.”

Sen. Bernie Sanders, en route from Toledo, Ohio, to Chicago to address a rally of his own, expressed concern over the incident.

“I hope that we are not in a moment in American history where people are going to be intimidated and roughed up and frightened about going to a political rally. ... I hope Mr. Trump speaks out forcefully and tells his supporters that that is not what the American political process is about.”

One of Trump’s GOP competitors, Ohio Gov. John Kasich, blamed Trump for the violence Friday.

"Tonight the seeds of division that Donald Trump has been sowing this whole campaign finally bore fruit, and it was ugly," Kasich said in a statement. "Some let their opposition to his views slip beyond protest into violence, but we can never let that happen. I urge people to resist that temptation and rise to a higher level."

Sen. Marco Rubio (R-Fla.) stopped short of blaming Trump but said the Republican front-runner is learning that "words have real consequences."

"I wouldn’t say Mr. Trump is responsible for the events of tonight," Rubio said on Fox News, "but he is most certainly, in other events, has in the past used some pretty rough language, saying in the good old days we used to beat these people up, or I’ll pay your legal bills if you rough them up. So I think he bears some responsibility for the general tone."

Republican presidential candidate Ted Cruz blames Donald Trump's campaign for disruptions at a rally in Chicago. (Reuters)

Sen. Ted Cruz (R-Tex.) organized an impromptu news conference outside a dinner event in Rolling Meadows, Ill., and took a harsher tone against Trump.

"The responsibility for that lies with protesters, who took violence into their own hands. But in any campaign, responsibility starts at the top," Cruz said. "Any candidate is responsible for the culture of a 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 discord."

Jose DelReal, Philip Rucker, John Wagner, David Weigel and Katie Zezima contributed to this report. Berman reported from Washington.