Hillary Clinton mercilessly mocked Donald Trump Thursday afternoon during a foreign policy address in San Diego, calling the presumptive Republican nominee a "dangerous" and "thin-skinned" candidate who is "temperamentally unfit" to be president.

Clinton's comments against Trump were her most forceful yet, and come as she prepares to clinch the Democratic nomination and officially begin the general election.

Her speech, at times, felt like a roast of the billionaire businessman, whom she called "dangerously incoherent," and whose foreign policy plans are "just a series of bizarre rants, personal feuds and outright lies."

She added that it would be a "historic mistake" to give "someone thin-skinned and quick to anger who lashes out at the smallest criticism" access to the nuclear codes.

"He says he doesn't have to listen to our generals or admirals, our ambassadors and other high officials, because he has 'a very good brain.' He also said, 'I know more about ISIS than the generals do. Believe me,'" Clinton said. "You know what? I don't believe him."

Trump began lashing out at Clinton during her remarks, saying she gave a "[b]ad performance" and didn't "look presidential."

But Clinton, during her speech, already had a line about Trump's penchant for Twitter rants teed up — undercutting his retort.

"We all know the tools Donald Trump brings to the table: bragging, mocking, composing nasty tweets. I'm willing to bet he's writing a few right now," Clinton said, at almost the exact moment Trump fired off his missive.

In one of her most forceful objections to Trump's temperament, Clinton attacked Trump's comments about dictatorial leaders in China, North Korea and Russia, saying "I will leave it to the psychiatrists to explain his affection for tyrants."

"Making Donald Trump commander-in-chief would be an historic mistake," Clinton concluded in her remarks, which got a standing ovation.