Donald Trump has called for a disarming of the bodyguards who protect his Democratic rival Hillary Clinton, and mused about the consequences of such a move by saying "let's see what happens to her".

The Republican presidential nominee was speaking at a rally in Miami, where he contrasted his supporters, who he said back police and want crime reduced, to Mrs Clinton, who he derided as someone who "lives behind walls and raises money from hedge funds".

"I think that her bodyguards should drop all weapons. They should disarm. I think they should disarm immediately, what do you think, yes?" he said.

"Take their guns away, she doesn't want guns. Take them, let's see what happens to her. Take their guns away, OK. It will be very dangerous."

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Both candidates have been protected by the Secret Service for months, but Mr Trump's latest take on Mrs Clinton's security detail brought swift denunciations, particularly from Mrs Clinton's allies.

"Tonight, Donald Trump once again alluded to violence against Hillary Clinton," said Elizabeth Shappell, spokeswoman for Correct The Record, a pro-Clinton media watchdog group.

"This is a truly deplorable comment that betrays our nation's most fundamental democratic values," Ms Shappell said in a statement.

Stuart Stevens, a Washington-based political consultant who worked on Republican Mitt Romney's 2012 presidential campaign, tweeted: "The Secret Service should investigate this threat."

'Second amendment people' can stop Clinton appointing judges

Mr Trump made a similar comment about Mrs Clinton and her armed protection in May while accepting the endorsement of the National Rifle Association, when he said Mrs Clinton would end the US Constitution's Second Amendment, which guarantees the right to bear arms.

"Let's see how they feel walking around without their guns or their bodyguards," he told the gun lobby group, speaking about Mrs Clinton and her Secret Service detail.

Mr Trump was criticised by opponents last month when he suggested that gun rights activists could act to stop Mrs Clinton from nominating liberal US Supreme Court justices, a comment some interpreted as encouraging a political assassination.

"If she gets to pick her judges, nothing you can do folks," Mr Trump told a rally in North Carolina on August 9.

"Although the Second Amendment people, maybe there is, I don't know," he continued.

Mrs Clinton has called for tighter access to guns, including universal background checks, but has never said she planned to get rid of the Second Amendment.

Trump backflips on 'birther' claims

Mr Trump's latest comments came just hours after he admitted that US President Barack Obama was born in the USA, reversing his apparent position after years of fuelling conspiracy theories.

"President Barack Obama was born in the United States, period," Mr Trump said, as his campaign tried to silence damaging charges of racism.

Sorry, this video has expired Donald Trump admits Barack Obama was born in the US.

Mr Trump pointedly did not apologise, and repeated allegations Mrs Clinton started the so-called "birther" movement questioning Mr Obama's nationality and therefore legitimacy as president.

"Her campaign of 2008 started the birther controversy. I finished it. I finished it. You know what I mean," Mr Trump said.

Mrs Clinton's campaign dismissed the claim as an outright lie.

Since at least 2011 Mr Trump has questioned Mr Obama's US citizenship, which is a legal prerequisite for becoming president.

In 2012 he offered $US5 million to anyone who could determine if President Obama was born in Hawaii or not.

The unfounded claims that Mr Obama was not a citizen became a launch pad for Mr Trump's political career, propelling him onto the national stage and winning him fans on the far right.

But in a tight election race, Mr Trump's position has become a liability, repulsing black, Hispanic and moderate voters who he needs to win the Oval Office.

Reuters/AFP/ABC