But US Secret Service spokesman Ed Donovan said these incidents posed no threat to the president's security.

'We pay attention to this obviously ... to someone with a firearm when they open carry even when they are within state law.

'We work with our law enforcement counterparts to make sure laws and regulations in their states are enforced,' he said.

But Obama supporters were clearly by perturbed by Monday's incident.

'It is extremely disturbing that you have that kind of weapon in close proximity to where the president is,' Ruben Gallego, a retired military veteran and Arizona Democratic Party official, was quoted as saying.

'He was demonstrating his Second Amendment rights, but he was clearly there to intimidate people who were there exercising their First Amendment rights,' Gallego said.

Obama is a target of about 30 potential death threats every day, according to a recent book.

Since Obama took office, threats to the US president have increased 400 per cent since President George W. Bush, says Ronald Kessler, author of 'In the President's Secret Service.'

Late last year, White supremacists in Tennessee had plotted to loot a gun store, kill 88 black people, decapitate another 14 and then shoot Obama.