PHOENIX — A town hall for Democratic presidential candidates turned chaotic on Saturday when protesters affiliated with the Black Lives Matter campaign took over the stage and confronted two prominent 2016 hopefuls.

Former Maryland Gov. Martin O’Malley and Vermont Sen. Bernie Sanders had come to the liberal Netroots Nation conference, held this year in Phoenix, Arizona, to address a largely sympathetic crowd of progressive activists, operatives, and communicators. Each candidate was to be interviewed one-on-one by journalist Jose Antonio Vargas; O’Malley first, then Sanders immediately afterward.

But Vargas did not get very far into his questioning of O’Malley before a crowd of Black Lives Matter activist marched through the conference hall ballroom and up to the stage, chanting, “What side are you on?"

Conference attendees then watched as two of the protesters —Black Alliance for Just Immigration national coordinator Tia Oso, followed by Black Lives Matter co-founder Patrisse Cullors — came onto the stage and spoke at length about Black Lives Matter. O’Malley stood by quietly.

“What will you do to stop police unions from battering our names after law enforcement kills us?” Cullors asked O’Malley. “I want to hear concrete actions. I want to hear an action plan. That's what we want to hear. And we want to hear it from Bernie Sanders too."

Cullors and Oso soon departed the stage, but protesters remained nearby for the duration of the event, sometimes chanting slogans and the names of black people who have died in police custody. Both O’Malley and Sanders seemed to founder in their response to the protests.

“Black lives of course matter,” said a visibly annoyed Sanders in response to heckling, early into his scheduled appearance after O’Malley. “But I’ve spent 50 years of my life fighting for civil rights. If you don’t want me to be here, it’s okay."

O’Malley, near the end of his own appearance, inspired boos from the protesters when he said, “Black lives matter, white lives matter, all lives matter.” He later told MSNBC’s Krystal Ball he had not been aware of the connotations behind the phrase, “all lives matter,” which has been adopted by many opponents of the Black Lives Matter movement.

Ashley Yates, a St. Louis, Missouri activist affiliated with Black Lives Matter, told Al Jazeera the protest was partially intended to draw attention to cases like that of Sandra Bland, a young black woman who was found dead in police custody earlier this week in Texas.

“We’re doing what we’ve been doing for the past 10 months, especially standing up for black life, but today with a real focus on black women — Sandra Bland, and saying her name,” said Yates. “And we’re bringing it to the presidential candidates. They claim to represent all of America, but then we get up there and you see that when they’re pressured on issues that are specifically black, they fumble."

The town hall’s audience appeared divided between those who cheered on the protests and those who were baffled and irritated by the interruption. Netroots Nation’s leadership attempted to strike a balance between giving credence to the protesters’ grievances and calling for order.

“Although we wish the candidates had more time to respond to the issues, what happened today is reflective of an urgent moment that America is facing today,” said Netroots Nation executive director Raven Brooks said in a statement.