'Black Lives Matter': Elizabeth Warren lashes out at racism and Republicans who 'cower' at Donald Trump

Elizabeth Warren threw down the gauntlet with an impassioned speech at Netroots Nation on Friday morning, criticizing mass incarceration and bashing Republicans for racism.

"Now look, House Republicans may still want to fly the Confederate flag, and Republican leaders may cower in the shadow of Donald Trump," she said. "But the American people understand that black lives matter and America is not a country that stands for racism, bigotry or hatred."

It is a moral and economic imperative to "build an economy that creates real opportunity, that doesn't lock up millions of our fellow human beings behind bars," she said.

"America must prove that on equality and justice, the American people are progressives," she said.

The Massachusetts senator was a speaker at the progressive convention held annually in Phoenix, Arizona.

Watch Warren's speech here: