A day of noisy but largely peaceful protests against United States President Donald Trump's speech in Phoenix turned unruly as police fired pepper spray at crowds after a protester apparently lobbed rocks and bottles at officers.

Trump, in his speech on Tuesday, mounted an aggressive defense of his response to a deadly far right march in Charlottesville, using the rally speech to condemn “dishonest” media coverage of his widely criticised remarks.

The disturbance started soon after Trump completed his speech.

Protesters and police clashed outside the convention center where Trump had delivered his speech. 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.

Four people were arrested on charges related to the protest, while another was arrested on an unrelated warrant, Police Chief Jeri Williams said. Two officers were treated for heat exhaustion, she said.

A small number of protesters stayed behind after police ended the clash.

Earlier, authorities had been put on high alert as thousands of people lined up in the triple-digit heat to attend Trump's first political rally since the violence in Charlottesville, Virginia. Protesters had 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 sauntered through the lane between the sides.

Local authorities were vigilant in the aftermath of the deadly protests in Charlottesville and the president's comments last week about both sides having blame for the violence at the white supremacist rally. Mayor Greg Stanton had unsuccessfully called on the president to not hold the rally here so soon after the trouble in Charlottesville.

“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 Trump and his backers about his election rival Hillary Clinton.

Dillon Scott of Phoenix, who voted for Clinton, said he came out to express dissatisfaction with how long Trump took to denounce racism after the Charlottesville violence.

A demonstrator is affected after police used tear gas to break up protesters.—AFP

“No one should be allowed to get away with what he gets away with, especially in a political office,” Scott said.

Meanwhile, a group of protesters chanted, “Wrong side of history! Wrong side of history!” 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,” Hutson said.

“He says what needs to be said,” she added.

A number of opposition signs showed drawings or photos of 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.

As the line to get in the venue moved ahead, the two groups shouted at each other, and some skirmishes broke out. At one point, a Trump supporter and a protester shoved each other.

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,” Brown said.

The outdoor temperature remained over 100 degrees as the rally began.

Capt. Rob McDade, the spokesman for Phoenix Fire Department, said that as of 6pm (local times) 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.

State Democratic leaders urged people who wanted to show their opposition to the president's policies to gather at a city-designated free speech zone near the site of the rally. The message to protesters echoed those coming from law enforcement and Stanton. Stanton said he expects protesters to be “civil, respectful and peaceful.”

Williams, the police chief, said First Amendment rights will be supported, but criminal conduct will be swiftly addressed. Some of Trump's supporters were so keen to see the president that they began queuing up before dawn for the 7pm rally.

“It's been on a bucket list of mine since he became the president,” said Kingman resident Diane Treon, who arrived at 4am. “I wished I had attended one of his campaign rallies before he became president, and I wanted to go to the inauguration. And truthfully it was the protests that kept me away” she added.

Treon said she wishes protesters “would be a little more peaceful instead of violently rioting, which is happening in so many places” but isn't overly worried.

Trump lashes out at media in Arizona rally

Trump faced bipartisan outrage after blaming “many sides” for violence at the rally in Charlottesville that took the life of an anti-fascist protester.

Re-reading his statements following the clashes in Tuesday's speech, he railed at reporters at the rally for misrepresenting his remarks — but omitted the equivocation that had sparked the backlash in the first place.

“The very dishonest media... and I mean truly dishonest people in the media and the fake media, they make up stories. They have no sources in many cases. They say 'a source says' — there is no such thing,” he said.

“But they don't report the facts. Just like they don't want to report that I spoke out forcefully against hatred, bigotry and violence and strongly condemned the neo-Nazis, the white supremacists and the KKK,” he added.

Trump dedicated around half an hour of his 78-minute speech to attacking the “sick people” in the news media, before turning his fire on his own side.

The speech was cheered on by supporters inside the conference center, though thousands of anti-Trump protesters, who had lined up under a blistering sun in Phoenix hours before Trump's arrival, later clashed with police outside the venue.