Trump's gun comments 'crossed the line', says Clinton

Hillary Clinton on Wednesday slammed her Republican presidential rival Donald Trump for his "casual inciting" of violence, saying his remarks suggesting gun rights supporters could act against her "crossed the line."

"Yesterday we witnessed the latest in a long line of casual comments by Donald Trump that crossed the line," Clinton, the Democratic nominee, told supporters in Des Moines, Iowa.

"Words matter, my friends. And if you are running to be president, or you are president of the United States, words can have tremendous consequences."

Trump on Tuesday caused a firestorm, including among some Republicans, when he suggested that "Second Amendment people" - those who support gun rights - could take action to stop Clinton from appointing US Supreme Court justices as president.

His campaign quickly sought to douse the flames, insisting the Republican flagbearer was merely urging gun rights supporters to reject her candidacy at the ballot box.

Clinton appeared to reject the Trump campaign's defence, warning of the dangers of reckless language during a presidential race.

She slammed Trump's "casual cruelty to a gold star family," referring to the brash billionaire's clash with the parents of a Muslim American soldier who was killed in action.

"His casual suggestion that more countries should have nuclear weapons, and now his casual inciting of violence," she added.

"Every single one of these incidents shows us that Donald Trump simply does not have the temperament to be president and commander in chief of the United States."

Moments before Clinton's remarks, a man later described by US media as an animal rights activist appeared to try to force his way on stage, but was stopped and removed by US Secret Service.

Agents rushed onto the stage to protect the candidate, but retreated after the protester was escorted out.

Agencies contributed to this report.