Police in Calfornia say they may have thwarted a third assassination attempt on Donald Trump after arresting a man with a loaded gun outside one of the former president's rallies.

Vem Miller, 49, was charged with being in possession of a loaded firearm and a high-capacity magazine at a security checkpoint near the Republican presidential nominee's rally at Coachella over the weekend.

It's not the first time authorities have moved to protect the former US president.

In July, a bullet grazed Mr Trump's ear at a rally in Pennsylvania, and in September the Secret Service found a man hiding by the former president's Palm Beach golf course with a rifle.

What happened?

Just before 5pm on Saturday (local time), police in Riverside County arrested Mr Miller at a security checkpoint outside a Donald Trump rally in Coachella, California.

Riverside County Sheriff Chad Bianco said Mr Miller was initially allowed through an outer perimeter checkpoint because he identified himself as a journalist.

But he said he was stopped at an inner perimeter checkpoint — closer to the rally — where officers inspected his black SUV and found that it was fitted with a "homemade" licence plate.

"He approached the outside perimeter, gave all the indications that he belonged there … and so he was allowed through that outer perimeter," Mr Bianco said.

"As he got to the inside perimeter where deputies were conducting obviously a more thorough evaluation of the vehicles that were coming in, there were many irregularities that popped up."

Sheriff Bianco said Mr Miller's car was in "disarray" and officers found multiple passports with different names, a loaded firearm and ammunition.

Mr Miller was released on $US5,000 ($7,400) bail to appear in court on January 2, 2025.

Who is Vem Miller?

Vem Miller is a registered Republican who holds a master's degree from the University of California, Los Angeles, according to the Southern California News Group.

The news outlet said he ran in the Nevada Republican state primaries in 2022 but lost.

In 2022, Mr Miller reportedly told the Las Vegas Review-Journal that he was motivated to run for office in Nevada because: "This country has been taken over by tyranny."

The publication described him as "a film and TV industry professional" who had experience producing investigative reports and managing budgets.

Mr Miller runs The America Happens Network, according to the Southern California News Group, which pledges to "rage against the mainstream media".

Sheriff Bianco said that Mr Miller was part of a group of individuals called "sovereign citizens".

He told the Southern California News Group that he believed Mr Miller was aligned with a "right-leaning anti-government group".

But in an interview with the organisation, Mr Miller said he was "shocked" that he was arrested and accused of trying to harm Trump.

Mr Miller denied that he had connections to the sovereign citizens group, according to the report.

He said he was a Trump caucus captain and received a special invitation to the Coachella Valley rally from the head of Clark County's Republican Party, according to the report.

What did police say?

Sheriff Bianco said his officers were working with the Secret Service and FBI to make sure they had "all of the information that they can gather".

"From my perspective … the firearms charges is what we arrested him for and booked him on. Anything further will come from the federal government," he said.

He defended his department's action to arrest the man on firearms charges.

"By an act of what we did in the week leading up to keeping that place secure, I certainly wouldn't want to be saying after the fact that 'I wish we could have done something to prevent that shooting'.

"We get to talk about it before.

The US Attorney's Office for Los Angeles said in a statement, citing the Secret Service, that Mr Trump was not in danger, no federal arrest had been made and an investigation was ongoing.