A forum featuring Democratic presidential candidates Martin O'Malley, former governor of Maryland, and Sen. Bernie Sanders, was interrupted by civil-rights activists during their appearances at the Netroots Nation conference in Phoenix on Saturday, according to The Huffington Post.

The protestors heckled O'Malley as he was being interviewed on stage, protesting against cases of police brutality and the treatment of African-Americans by law enforcement.

The protestors marched into the conference hall with chants of "Black lives matter!" One of the group's leaders climbed onto the stage and grabbed the microphone to address the audience about the police-involved shootings, the treatment of immigrants and Arizona's racial history, as O'Malley and interviewer Jose Antonio Vargas looked on helplessly, according to Fox News.

O'Malley left the stage with a departing comment "Black lives matter. White lives matter. All lives matter," which was met with some heckles and boos from the crowd.

The former Maryland governor later apologized for his comment. "I meant no insensitivity by that and I apologize if that's what I communicated," he said. "That was misstated. What I intended to say was that we're all in this together - that black lives do matter and we have a double-standard of justice in this country," according to CNN.

Sanders was also shouted down by protestors as he tried to address the audience. At one point, Sanders stated "Black lives of course matter. I spent 50 years of my life fighting for civil rights and if you don't want me to be here, that's OK."

During his 20-minute appearance, Sanders talked about the need to address issues such as wealth and income inequality, adding that African-Americans and Hispanics face high rates of unemployment. Sanders was also scheduled to have a meeting with representatives of the organization Black Lives Matter following his speech, but his campaign manager, Jeff Weaver, attended the meeting instead, according to spokesman Michael Briggs.