Hillary Clinton has called republican presumptive nominee Donald Trump “dangerously incoherent”.
The democratic front-runner said Mr Trump was unfit to be president and his election would be a “historic mistake”.
In her speech Mrs Clinton, who is fighting Bernie Sanders for the democratic nomination, rejected Mr Trump’s foreign policy points, and called him thin-skinned, irrational and unprepared.
Mr Trump hit back, saying Mrs Clinton “no longer has credibility – too much failure in office”.
Outside Mr Trump’s rally in San Jose, California, anti-trump protesters clashed with his supporters in one of the worst such confrontations so far.