Days after a failed assassination attempt on former American President Donald Trump, Republican Party presidential candidate, a man with a gun in a backpack was arrested at the Republican National Convention.

“Police initially observed the suspect looking suspicious, wearing a ski mask and a large tactical backpack in the street,” U.S. Capitol Police, Milwaukee police said on Tuesday.

The police said the 21-year-old man, who did not have a permit to carry a firearm, was arrested two blocks outside the vehicle screening perimeter of the GOP convention.

The four-day convention, which began on Monday, saw Mr Trump in attendance after narrowly escaping an assassination attempt during his campaign in Butler, Pennsylvania, on Saturday. 

Thomas Matthew Crooks, a 20-year-old from Pennsylvania, had opened fire on Mr Trump, injuring him in the ear, killing one person, and leaving two others wounded. 

This latest arrest at the GOP convention came amid an ongoing investigation into the failed assassination attempt on Mr Trump.

On the convention’s first day, Mr Trump was officially ratified as the GOP presidential candidate for the 5 November election. On the same day, he announced Ohio Senator James David Vance as his running mate.