After generating positive buzz at an Iowa Democratic dinner, former Maryland Gov. Martin O'Malley was booed and heckled at a progressive conference Saturday for saying all lives matter, according to multiple reports.

Both 2016 Democratic presidential candidates who spoke to the annual Netroots Nation conference, O'Malley and Bernie Sanders, were interrupted by protestors concerned about the impact of police brutality on the black community.

"Say that black lives matter," demonstrators challenged them, the Hill reported. "Say that I am not a criminal. Say my name."

The moderator allowed a protest leader to question O'Malley. "We are in a state of emergency," she said. "If you do not feel that emergency, then you are not human. I want to hear concrete action plans."

"I think all of us have a responsibility to recognize the pain and grief caused by lives lost to violence," O'Malley replied. "Black lives matter. White lives matter. All lives matter." The Hill described this response as "drawing boos from the demonstrators."

Some activists have complained about the phrase "all lives matter," going so far as to call it an "inappropriate, racist response to Black Lives Matter."

"Non-black kids aren't being killed like black kids are," wrote Julia Craven in Huffington Post. "Of course I'd be just as pissed if cops were gunning down white kids."

Sanders had his own dust-ups with chanting protestors. Saying he didn't want to "shout over people," the socialist senator from Vermont said, "Black lives of course matter. I have spent 50 years of my life fighting for civil rights."

The incidents highlight the difficulty progressive challengers to Clinton are going to have if they can't broaden their bases beyond white liberals. O'Malley has come under fire for his record as mayor of Baltimore after young black man Freddie Gray died in police custody. Sanders has represented one of the whitest states in the country in Congress since the early 1990s.

Barack Obama defeated Hillary Clinton by a narrow margin in the 2008 Democratic primaries because he was able to take supermajorities of black voters away from her, joining them with the white liberal voters who had previously been drawn to losing candidates like Howard Dean.

The Netroots Nation conference is taking place in Phoenix, Ariz.