Democratic presidential nominee Hillary Clinton has said Donald Trump incited violence when he said gun rights supporters could stop her from winning.
Speaking at a rally in Des Moines, Iowa, Mrs Clinton said “words can have tremendous consequences.”
Republican nominee Mr Trump sparked a backlash after suggesting “Second Amendment people”, or gun owners, could take action against her.
Mrs Clinton also said Mr Trump did not have the temperament to be president.
The former secretary of state criticised Mr Trump for his recent row with the family of a fallen American Muslim soldier, which the military refers to as a Gold Star family.
“Yesterday we witnessed the latest in the long line of casual comments from Donald Trump that cross the line. His casual cruelty to a Gold Star family, his casual suggestion that more countries should have nuclear weapons. And now his casual inciting of violence,” she said.
“Every single one of these incidents shows that Donald Trump simply does not have the temperament to be president and commander in chief of the United States.
Mr Trump was speaking at a rally in North Carolina on Tuesday when he said Mrs Clinton would put liberal justices on the Supreme Court if she wins the presidency.
The Republican nominee suggested her liberal nominations would threaten gun ownership rights when he said: “Hillary wants to abolish, essentially abolish the Second Amendment. By the way, and if she gets to pick her judges, nothing you can do, folks.
“But the Second Amendment people, maybe there is, I don’t know.”
Social media users swiftly responded, condemning Mr Trump for appearing to incite violence.
Mr Trump dismissed the claims, tweeting that he was trying to unify gun rights supporters to turn out to vote to defeat Mrs Clinton.
His campaign said: “Second Amendment people have amazing spirit and are tremendously unified, which gives them great political power.”
“And this year, they will be voting in record numbers, and it won’t be for Hillary Clinton, it will be for Donald Trump.”
Meanwhile, former Nato Secretary General Anders Fogh Rasmussen said a victory for Mr Trump in the November election would undermine the credibility of the US.
A Trump presidency would signal the end of “the American-led world order”, Mr Rasmussen – a former Danish prime minister who was Nato chief between 2009 and 2014 – told the Politico website.
Mr Trump has criticised US interventions abroad, notably in Syria, and has been accused of isolationist tendencies.