The largely peaceful protests at the President’s speech in Phoenix descended into chaos as police launched pepper spray and tear gas at crowds of activists.
Protesters and police clashed outside the convention centre where Mr Trump has just wrapped up his speech, with some demonstrators apparently launching rocks and bottles at officers.
People fled the scene coughing as an officer in a helicopter bellowed through a speaker urging protesters to leave the area.
Officers responded with pepper spray to break up the crowd after people tossed rocks and bottles and dispersed gas, Phoenix police spokesman Jonathan Howard said.
It was not immediately known if there were injuries or arrests.
Authorities were on high alert as thousands of people lined up to attend Mr Trump's first political rally since the violence in Charlottesville, Virginia.
Protesters filled the streets of downtown Phoenix and engaged in shouting matches and a few minor scuffles with Trump supporters, but those events were generally peaceful.
By the time Trump took the stage, police said there had been no arrests or major incidents.
Phoenix police kept most members of the two opposing groups behind barricades and apart on separate sides of the street.
Officers wearing riot gear and carrying rifles walked through the lane between the two sides.
"Toxic Trump," read one protest sign held up to the president's supporters streaming into the Phoenix Convention Center downtown. "Lock Him Up!" read another, a reference to earlier campaign chants by Mr Trump and his backers about his election rival Hillary Clinton.
Dillon Scott of Phoenix, who voted for Mrs Clinton, said he came out to express dissatisfaction with how long Mr Trump took to denounce racism after the Charlottesville violence.
"No one should be allowed to get away with what he gets away with, especially in political office," Mr Scott said.
Trump backer Randy Hutson, a retired Phoenix police officer, began standing in line more than seven hours before the speech was to start.
"He is the first president I feel in my lifetime that speaks his mind and speaks from the heart," Mr Hutson said. "He says what needs to be said."
A number of opposition signs showed drawings or photos of Mr Trump with a small, Hitler-style mustache. Three Trump supporters taunted Latino protesters with offensive comments about immigrants and held anti-Muslim and Black Lives Matter signs.
John Brown, of an anti-Trump group calling itself the Redneck Revolt, wore military fatigues and had an AK-47 rifle strapped to his chest as he and a half dozen others from the group stood about 30 feet behind the barricade for protesters.
He said they were there to protect Trump opponents and stand up to fascism. "He's offensive to me in every way," Mr Brown said.
The disturbances began after the speech had ended and a crowd of protesters stayed behind.
Local authorities were vigilant in the aftermath of the deadly protests in Virginia and the president's comments last week about both sides having blame for violence at the white supremacist rally.
Mayor Greg Stanton had begged the president to not hold the rally here so soon after the trouble in Charlottesville.
The outdoor temperature remained at nearly 38 degrees as the Phoenix rally began.
Captain Rob McDade, spokesman for Phoenix Fire Department, said that as of 6 pm they had treated 48 people for heat-related problems, most of them for dehydration. Of those, two were adult women who were taken to a hospital for further evaluation, he said.