Hillary Clinton has described Donald Trump, the presumptive Republican nominee, as "dangerously incoherent" and accused him of peddling "outright lies".
The Democratic presidential hopeful criticised her election rival as never before on Thursday, questioning Trump's foreign policy and calling the him "unfit for office".
"He is temperamentally unfit to hold an office that requires knowledge, stability and immense responsibility," Clinton told an audience in San Diego, California, escalating the rhetoric in what is already a deeply acrimonious election season.
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"Donald Trump's ideas aren't just different - they are dangerously incoherent," she said.
"They're not really ideas, just a series of bizarre rants, personal feuds and outright lies.
"This is not someone who should ever have the nuclear codes."
Democratic and Republican presidents have been generally consistent on policies affecting China, Russia, North Korea, nuclear proliferation, trade, alliances and many other issues but Clinton and Trump offer starkly different visions of US foreign policy.
Grasp of world affairs
Trump has questioned the utility of NATO, appeared indifferent to the prospect of nuclear weapons proliferation and expressed admiration for Russian President Vladimir Putin and North Korean leader Kim Jong-un.
"I will leave it to the psychiatrists to explain his affection for tyrants," Clinton said at her first major foreign policy address, while trying to highlight her own grasp of world affairs.
"Even if I weren't in this race, I'd be doing everything I could to make sure Donald Trump never becomes president because I believe he will take our country down a truly dangerous path."
Clinton, who formerly served as secretary of state, first lady and US senator, is seeking to become the first female president in the United States' 239-year history.
For his part, Trump, speaking at a rally in San Jose, California, on Thursday night ridiculed Clinton's speech.
"It was pathetic. It was so sad to watch," he said.
"She was up there, supposed to be a foreign policy speech, it was a political speech, had nothing to do with foreign policy."
Paul Ryan's backing
Clinton and Trump are all but assured to be the Democratic and Republican party nominees.
In a boost for Trump, Paul Ryan, the Speaker of the House of Representatives, said that he would back his candidacy.
But even as Trump wins the backing of Republicans in Congress, many party leading lights who take pride in Republicans' traditionally strong national security credentials are hostile to Trump leading the party.
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Trump has angered many with his rhetoric against Hispanics, women, Muslims and others.
At a rally in San Jose, Trump's critics scuffled with his supporters as fistfights erupted and one supporter was hit by an egg.
Crowds had earlier chanted "No hate in our state" and carried signs that read "Dump Trump" as they marched near the convention centre where Trump was speaking.
Americans will go to the polls on November 8 to elect a new president, replacing Barack Obama who steps down after eight years in office.
Source: Al Jazeera and agencies