Republican presidential front-runner Donald Trump brought his message of political indignation to a packed house at Costa Mesa’s 8,000-seat Pacific Amphitheatre on Thursday and was welcomed by a cheering crowd for providing an alternative to the status quo.

Within minutes of taking the stage to kick off his California campaign, Trump had the crowd on its feet chanting, “Build that wall.”

• Photos: Donald Trump rally in Orange County

“Your crime numbers, they’re going through the roof, and we can’t have it anymore,” Trump told the packed venue at the OC Fair & Event Center. “We’re going to get our country back to a balance.”

Outside, sheriff’s deputies on horseback and in riot gear had to separate pro- and anti-Trump groups who shouted profanities at each other and nearly came to blows, with one side chanting “Dump Trump” as the other shouted, “Go back to Mexico.” As Trump spoke, a large crowd of protesters, some waving Mexican flags, blocked an intersection outside. Drivers burned rubber, filling the air with smoke.

The tensions may be an early sign of what’s ahead for California in the weeks leading up to the June 7 primary, as Trump shifts his attention to the state that could prove the most crucial yet in his drive to be the Republican standard bearer.

“No state has suffered more from open borders than the state of California,” Trump told the crowd.

Trump’s choice of Orange County for his first major California event of the year is no surprise. While the county’s increasing ethnic diversity has contributed to Republican voter registration recently slipping below 40 percent, the GOP still has an 8-point advantage over Democrats and continues its long-standing national reputation as a Republican powerhouse.

The county’s many wealthy donors also make it a national fundraising hub for GOP candidates.

Hours before the rally began, large crowds of flag-waving supporters and scattered sign-carrying protesters gathered at the fairgrounds.

Dawn Mayo stood on a concrete planter box in front of the Pacific Amphitheatre, surrounded by Trump supporters. She waved a blue “Make America great again” hat as she tried to lead the crowd in a “Go Trump!” chant that quickly faded.

“I’ll get them excited. Give me time,” said Mayo, 49, who grew up in New York and drove from San Diego on Thursday on afternoon to attend the rally. “I love Trump. I want the energy to be up and people to be as excited as I am.”

A first clash came around 4:30 p.m.

“We don’t want you here,” part of the crowd chanted at a Latino-American man, Juan Rodriguez, who wore an anti-Trump shirt and waved a Mexican flag. The 20-year-old Bernie Sanders supporter said he came to the rally to exercise his First Amendment rights.

A young man in a suit and a red hat, carrying a flag, crept into the scrum and went toe-to-toe with Rodriguez. They screamed profanities at each other. “We want a wall to protect ourselves,” the man in the hat said.

“Trump won’t make anything better,” Rodriguez responded.

The atmosphere outside became increasingly tense as the rally’s scheduled start time approached.

About three dozen protesters marched toward Trump supporters who were waiting in line to get in. Sheriff’s deputies acted quickly — as both sides shouted slurs and profanity — creating a barrier between the two sides. Some officers were on horseback.

Those on the pro-Trump side were told to stand on the sidewalk.

“It was scary,” said Chelsea Rogers, 25, a Costa Mesa cosmetologist who came to the rally with her 16-year-old brother and who supports Trump. “If it gets any crazier, I don’t know what the police will do.”

On the anti-Trump side, Katrina Mendoza, 22, an Orange Coast College student, quickly walked away from the crowds as her friends urged her not to go back to the protest.

“A lady tried to hit me,” Mendoza said. “She called me disgusting and told me to go back to my country. But I was born here.”

Both sides continued shouting obscenities, with hundreds of protesters cordoned off in a portion of the parking lot.

Inside the amphitheater, the atmosphere more closely resembled a rock concert, with Elton John music blasting from loudspeakers as the crowd got ready for Trump to take the stage.

“Mr. Trump is not going to do this by himself. We are an army. Together, we can do anything,” said Rancho Santa Margarita Mayor Tony Beall, warming up the crowd.

An announcer over the loudspeaker instructed protesters to stay outside in a designated area. And he told Trump supporters they should not touch the protesters.

Secret Service agents escorted one man out as he filmed the crowd with a hand-held camera. It wasn’t clear what triggered his removal. Agents also took a T-shirt deriding Trump with an expletive from another man.

Meanwhile, many people in the crowd were talking politics among themselves.

David Rose, 58, of Cypress, a lifelong Republican, said Trump represents “average Americans” and has the business acumen to run the country.

“He’s independent, not beholden to special interests and has had success running large organizations,” Rose said.