PHOENIX, ARIZONA (WASHINGTON POST, BLOOMBERG, NYTIMES) - President Donald Trump on Tuesday (Aug 22) threatened to shut down the government over border wall funding, said the North American Free Trade Agreement is likely to be terminated and signalled that he was prepared to pardon former Arizona sheriff Joe Arpaio, who is anathema to the Latino community.

Trump's freewheeling comments came during a boisterous campaign rally here during which he also went on an extended diatribe about the media, blaming reporters for the negative fallout he has received over his responses to the hate-fueled violence in Charlottesville.

In a speech that stretched well over an hour, he delivered a warning to Democratic lawmakers who have objected to his plans to construct a wall along the US-Mexico frontier, calling them "obstructionists" and saying it was time for the US to crack down on illegal immigration.

"If we have to close down our government, we're building that wall," Trump told thousands of supporters gathered in Phoenix for a campaign-style rally. "One way or the other, we're going to get that wall."

Trump has asked for US$1.6 billion to begin construction of the wall, with Congress under pressure to pass some kind of spending bill to keep the government open after Sept 30.

But Republicans in Congress haven't shown much appetite for fighting to spend potentially billions more on a border barrier either. The funding would add to the deficit at the same time Republicans are trying to figure out how to pay for tax cuts.

Futures on the S&P 500 Index reversed gains to slip as much as 0.3 per cent as Trump spoke. The yen strengthened, while the Mexican peso weakened 0.2 per cent as the president also said he might terminate the North American Free Trade Agreement at some point.

"His comments on the NAFTA negotiations once again brings the general direction towards obstructing free trade, and raises concerns over its impact on global trade," said Hideyuki Ishiguro, a senior strategist at Daiwa Securities Co. in Tokyo.

At the outset of the rally, Trump selectively recounted the series of statements he made in the days following the melee in Charlottesville, arguing that he "spoke out forcefully against hatred and bigotry and violence" but that the media - whom he called "sick people" - refused to report it properly.

"You know where my heart is," Trump said, before pulling a copy of his first of three statements on the violence out of his suit coat and reading it to his audience. He later accused the media of giving a platform to the hate groups that were central to the violence in Charlottesville that led to three deaths.

"It's time to challenge the media," Trump said, for their role "in fomenting divisions". The president added: "The only people giving a platform to these hate groups is the media itself and the fake news."

Following his comments last week, Trump was criticised by Democrats and Republicans alike for blaming "both sides" for the violence and saying that "fine people" had marched along with white supremacists to protest the removal of a Confederate statue. He did not mention either of those remarks Tuesday.

The rally, organised by Trump's reelection campaign, came as the president continues to face criticism for his response to Charlottesville and feuds with fellow Republicans in Congress whose cooperation he will need to kick-start his sputtering legislative agenda next month.

Democrat Phoenix Mayor Greg Stanton had urged Trump to not come to his city this week, saying that it was too tense of a time in the wake of the deadly Charlottesville clash between white nationalists and counter-protesters and that Trump could be setting the stage for more violent strife here. He also said that a pardon of Arpaio could make the situation even more dire.

The Arizona sheriff was convicted last month of criminal contempt for ignoring a federal judge's order to stop detaining people because he merely suspected them of being undocumented immigrants. A major Trump supporter during last year's campaign, he awaits sentencing.

"So was Sheriff Joe convicted for doing his job?" Trump asked the crowd. "You know what, I'll make a prediction: I think he's going to be just fine, okay? But I won't do it tonight because I don't want to cause any controversy. But Sheriff Joe should feel good."

Trump last week told Fox News that he was "seriously considering" a pardon for Arpaio and said he might do it soon, sparking speculation he would use Tuesday's campaign rally here to make the move.

Inside a partially filled Phoenix Convention Centre, Trump was given a hero's welcome from supporters who chanted "USA! USA! USA!" and waved signs reading "Drain the Swamp", "Make America Strong Again" and "Make America Proud Again".

"You were there from the start, you've been there every day since, and believe me, Arizona, I will never forget it," Trump said at the start of his remarks, referencing a large crowd he drew at the site early in his campaign. His crowd Tuesday night numbered in the thousands but did not completely fill the hall at the convention centre.

There was a heavy police presence in downtown Phoenix, with law enforcement seeking to maintain civility between Trump supporters and detractors.

Police used gas and smoke canisters to disperse crowds as tempers flared. Hundreds of people ran off, streaming into the surrounding streets, coughing and wiping tears from their eyes. Police helicopters circled above downtown Phoenix after the speech, telling people to leave the area or face arrest.

While tensions were high before and during the speech - the police tried to keep supporters and opponents of the president apart outside - they escalated afterward. Jeri Williams, the chief of the Phoenix Police Department, said at a news conference late Tuesday that officers were attacked with bottles, rocks and tear gas and that two officers were being treated for heat exhaustion at a hospital.

She disputed the suggestion that officers were overly aggressive, saying they responded with tear gas and so-called pepper balls, which emit pepper spray, only after they were assaulted.

But some witnesses said that events unfolded differently, with protesters throwing a water bottle or two in the direction of the police, before the police fired tear gas into the crowd.

Mayor Stanton, who also spoke at the news conference, said that the police had attempted to allow people on downtown streets to protest peacefully and that there had been no serious injuries. But he added that officials were going to examine whether the approach by the police was necessary.

"There's going to be an after-incident review," Stanton said. Four people were arrested in connection with the rally, including two people charged with assaulting police officers, the authorities said.