What happened to Bernie Sanders and Martin O'Malley in Phoenix?

Democratic presidential candidates Bernie Sanders and Martin O'Malley arrived at the annual Netroots Nation convention hoping to impress some of the party's most influential liberal activists. Things didn't exactly go as planned.

Demonstrators protesting cases of police brutality and the treatment of black Americans by law enforcement disrupted a presidential forum Saturday as Mr. O'Malley, a former Maryland governor, was interviewed on stage. The group later heckled Sanders, a Vermont senator. Polls show both trailing Democratic front-runner Hillary Clinton, who did not attend the forum.

The raucous scene unfolded when a large group of protesters streamed into the convention hall chanting, "Black lives matter!" As O'Malley and interviewer Jose Antonio Vargas looked on, one of the group's leaders took over the stage and addressed the audience as the largely female group of demonstrators railed against police-involved shootings, the treatment of immigrants and Arizona's racial history.

Before departing, O'Malley told the convention: "Black lives matter. White lives matter. All lives matter," prompting some heckles and boos in the crowd.

He later apologized at an immigration event. "I meant no insensitivity by that and I apologize if that's what I communicated," he told reporters. "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."

Sanders tried to address the roughly 3,000 Netroots activists as many of the protesters shouted at him and disrupted his remarks. At one point, Sanders said: "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 an abbreviated 20-minute appearance, the self-described democratic socialist about the need to address wealth and income inequality, noting that blacks and Hispanics face high rates of unemployment.

Sanders later addressed police brutality at a large rally in Phoenix Saturday night, telling a crowd of more than 11,000 it is unacceptable for young black men to be beaten and killed while walking down the street.

"When a police officer breaks the law, that officer must be held accountable," Sanders said. He later quoted the 19th century black abolitionist Frederick Douglass: "Freedom doesn't come without struggle."

Sanders and O'Malley are vying to become the Democratic alternative to Clinton, who was campaigning in Iowa and Arkansas on Saturday. Sanders has risen in polls in recent months and sought to broaden his appeal to minorities and a more diverse section of the Democratic electorate, addressing immigration and criminal justice.

Yet some liberal groups panned the appearances. Anna Galland, the executive director of MoveOn.org Civic Action, said the responses showed that all Democratic candidates have work to do to understand the black lives movement.

Galland said while issues of economic and racial justice intersect, "portrayals of racial injustice as merely an offshoot of economic injustice or the implication that solutions to economic inequality will take care of racism represent a fundamental misunderstanding of how race operates in our country."

The demonstrators were promoting the national "Black Lives Matter" movement, which seeks changes to law enforcement policies following several high profile deaths of black men at the hands of police.

Clinton spoke up for raising the minimum raise during a dinner for state Democrats Saturday night in North Little Rock, Arkansas. She noted that "the economy is still stacked for those at the top."

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Associated Press writer Andrew DeMillo in North Little Rock, Arkansas, contributed to this report.