What started as a relatively subdued speech turned heated when Sen. Elizabeth Warren said to loud cheers that Donald Trump is "a man who must never be president of the United States."

Warren continued to rail against his "stupid" plan to build a wall and "goofy hat."

"Here's the really ugly underside to his pitch: Trump thinks he can win votes by fanning the flames of fear and hatred," Warren added. 

Bernie Sanders supporters throughout Warren's remarks were heard heckling the senator, exclaiming, "We trusted you!" Later, some were heard chanting "Goldman Sachs" and "You let us down."

Warren thanked Sanders and President Obama at the opening of her speech. 

To Republicans, Warren warned, "This November, the American people are coming for you."

"We are not going to be Donald Trump's hate-filled America," she said. "Not now. Not ever."

Warren in her closing remarks made a pitch for Hillary Clinton, saying, "If you believe we must reject the politics of fear and division, if you believe we are stronger together, then let’s work our hearts out to make Hillary Clinton the next President of the United States!"