At one point, Spencer said, "You know that what I am saying here will change the world." At another point, he described the audience as a mob.
People chanted, "Black lives matter!" and "Go home, Spencer!"
"Are you adults?" Spencer asked. "It doesn't look like it."
Spencer called the crowd "shrieking and grunting morons."
They responded by chanting, "Let's go, Gators!"
The public university spent an estimated $600,000 on security for the event.
More than 500 law enforcement officers were deployed, a state of emergency was declared, and many students avoided classes, and campus, entirely on Thursday.
With an intense police presence - snipers were positioned on the rooftops of nearby buildings, hundreds of uniformed state troopers stood at attention behind barricades - the protest outside the speech proved peaceful.
The event was Spencer's first public speech on a college campus since he led torch-bearing followers through the University of Virginia in August, the start of a weekend of clashes between white nationalists and white supremacists and counterprotesters that turned deadly in Charlottesville the next day.
Spencer's efforts to speak at UF had been closely watched and bitterly debated - a sign not only of how raw the tensions over race and culture remain but also of the intensity of the fight over free speech on college campuses.
The campus of 52,000 students was eerily quiet Thursday morning, with a heavy police presence, barricades and road closures, but by early afternoon, crowds of protesters had gathered to counter Spencer's appearance.
"We have a duty to fight for our freedom," a woman in an orange tank top shouted, leading a group of marchers who repeated her words in unison.
There was a brief scuffle when protesters turned on a man wearing a shirt that was branded with a swastika, and he was marched out of the crowd. But, mostly, people chanted in unison: "Not my town, not my state, we don't want your Nazi hate!"
When an airplane carrying a banner that read, "Love Conquers All! Love will prevail!" flew overhead, the crowd erupted in cheers.
Before he spoke inside a heavily secured performing arts center, Spencer answered questions at an often contentious news conference. He said it was "absolutely right" that the university and state expected to spend more than $600,000 on security. "This is the free-speech issue of our day."
Asked whether he was a racist, he said he was not a racist in a "cartoonish" sense but that, "Yes, race is real, race matters, and race is the foundation of identity."
Eight hundred tickets were handed out for the event, but the lower level of the Curtis M. Phillips Center for the Performing Arts looked to be only about half-filled moments before Spencer began his speech. A theater manager said there were about 400 people inside, including media. The protest and chants in the auditorium began as soon as the event began and continued until Spencer finally walked offstage 90 minutes later.
People came for many reasons.
"I came here to support Spencer because after Charlottesville, the radical left threatened my family and children because I was seen and photographed in Charlottesville," Tyler TenBrink, 29, said. "The man's got the brass to say what nobody else will."
Crew Kinnard, 58, a nurse from Gainesville, came to hear what Spencer had to say "because I want to know what I'm arguing against.
"I want to know what logic and what information he might be using," Kinnard said. "It breaks my heart that this is happening in the 21st century, but we all have freedom of speech."
Emmanuel Kizito, a 20-year-old political science major at UF, sat near the back of the auditorium with a group of black students. He said he "came to witness Spencer's violent rhetoric and to indict the University of Florida . . . who emboldened his ideals by allowing him to speak."
After Spencer's speech ended, the few supporters who did show up began to trickle out, and the protesters started shouting.
One man emerged from behind the police line only to be sprayed in the face with something, witnesses said. Police began escorting others.
"Nazi scum!" protesters screamed as one man in a white polo shirt and a slicked-down side part emerged from the theater, and state troopers formed a cordon between him and the protesters and helped him move down Hull Road. The crowd followed, chanting and surrounding them.
In a telephone interview after the speech, Spencer described the appearance as "frustrating and exhilarating at the same time."
"I'm inspired that we persevered against totally thuggish behavior," he said. "Screaming at the top of your lungs is the same as trying to bar the door."
Spencer called his appearance a "very big win for us and a very big loss for the University of Florida and antifa."
Zachary Bautista, a University of Florida medical student, said he views the protest as part of a larger series of demonstrations related to hate and injustice across the country. There was the women's march against President Trump and racism in Washington. There were the marches against racial inequality in Missouri.
Now, it's Gainesville's turn, he said.
"Having the presence of someone like Richard Spencer here is a call to action for us," Bautista, 23, said. "This is our opportunity to let everyone know we don't agree with this. We want everyone to know we want equality and opportunity and for everyone to get along."
Police on Thursday fenced off a vast parking lot adjacent to the artcomplex. Campus police, officers from the Florida Highway Patrol and other law enforcement agencies took up positions around the campus and the designated protest zone Thursday morning. Spencer and his opponents praised the law enforcement response.
All major roads leading to the event were blocked by dump trucks or other large vehicles.
Outside the barriers, a sign listed dozens of prohibited items: no firearms, tasers, fireworks, torches, masks or chains; no wagons or pull carts; no pets, no drones, no skateboards or laser pointers.
Police made two arrests. A 28-year-old man from Orlando was arrested for carrying a firearm on school property. And a 34-year-old man was charged with resisting arrest without violence.
Gov. Rick Scott, R, declared a state of emergency days before the speech. University officials sent out cautionary emails about "the event," as they called it, urging students to avoid the area and denouncing the "hateful rhetoric" of the National Policy Institute.
And protesters converged, blitzing social media. A group called No Nazis at UF urged solidarity on social media and offered detailed plans and shuttle rides to get as close as possible to the closed-off area.
Mike Ryan Simonovich, 39, a stay-at-home father from Gainesville, was trying to find tickets to see Spencer's speech. His plan was to attend the event and then walk out when Spencer started speaking to "demonstrate my contempt for his odious views."
Ryan Simonovich said he acknowledged that Spencer's right to speak was protected by the First Amendment.
"A roomful of angry liberals shouting at him does more to promote his ideas than people walking out in contempt does," he said.
Spencer's supporters had been planning, too. On the Daily Stormer, Andrew Anglin advised people to dress inconspicuously ("if you've got Nazi tattoos, cover them up"), avoid the designated protest area ("TRAP ZONE") and try to get a ticket to the speech.
Spencer is trying to keep the momentum going for his movement by appealing to college students, "trying to get young disaffected whites interested in white nationalism," as well as getting media attention, said Marilyn Mayo of the Anti-Defamation League's Center on Extremism.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)