Phoenix (CNN) Democratic presidential candidate Martin O'Malley apologized on Saturday for saying "All lives matter" while discussing police violence against African-Americans with liberal demonstrators.

Several dozen demonstrators interrupted the former Maryland governor while he was speaking here at the Netroots Nation conference, a gathering of liberal activists, demanding that he address criminal justice and police brutality. When they shouted, "Black lives matter!" a rallying cry of protests that broke out after several black Americans were killed at the hands of police in recent months, O'Malley responded: "Black lives matter. White lives matter. All lives matter."

From left, Samaira Rice, the mother of Tamir Rice; Lesley McSpadden, the mother of Michael Brown Jr.; the Rev. Al Sharpton and Michael Brown Sr., the father of Michael Brown Jr, raise their hands in the air during the "Justice For All" march and rally through Washington on December 13.

From left, Samaira Rice, the mother of Tamir Rice; Lesley McSpadden, the mother of Michael Brown Jr.; the Rev. Al Sharpton and Michael Brown Sr., the father of Michael Brown Jr, raise their hands in the air during the "Justice For All" march and rally through Washington on December 13.

A demonstrator marches in New York on December 13 during the "Justice for All" rally.

A demonstrator marches in New York on December 13 during the "Justice for All" rally.

Demonstrators march over the inbound lane of the Brooklyn Bridge in New York on December 13.

Demonstrators march over the inbound lane of the Brooklyn Bridge in New York on December 13.

Mikela Mosley speaks out in front of a line of police during a demonstration on Saturday, December 13, in Oakland, California.

Mikela Mosley speaks out in front of a line of police during a demonstration on Saturday, December 13, in Oakland, California.

Police line up to move the protesters from out of the mall on December 20 in Bloomington.

Police line up to move the protesters from out of the mall on December 20 in Bloomington.

Police arrest a "Black Lives Matter" protester on Saturday, December 20, in Bloomington, Minnesota. Invoking the familiar names of blacks who died at the hands of police, including Eric Garner, Michael Brown and Tamir Rice, thousands have taken part in protests across the country calling for a more aggressive federal response to recent slayings by police.

Police arrest a "Black Lives Matter" protester on Saturday, December 20, in Bloomington, Minnesota. Invoking the familiar names of blacks who died at the hands of police, including Eric Garner, Michael Brown and Tamir Rice, thousands have taken part in protests across the country calling for a more aggressive federal response to recent slayings by police.

The demonstrators, who were mostly black, responded by booing him and shouting him down.

Later that day, O'Malley apologized for using the phrase in that context if it was perceived that he was minimizing the importance of blacks killed by police.

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"I meant no disrespect," O'Malley said in an interview on This Week in Blackness , a digital show. "That was a mistake on my part and I meant no disrespect. I did not mean to be insensitive in any way or communicate that I did not understand the tremendous passion, commitment and feeling and depth of feeling that all of us should be attaching to this issue."

Judith Butler, a philosopher at the University of California, Berkeley, recently explained why some find it offensive to respond to the "Black Lives Matter" movement with the "all lives matter."

"When some people rejoin with 'All Lives Matter' they misunderstand the problem, but not because their message is untrue. It is true that all lives matter, but it is equally true that not all lives are understood to matter, which is precisely why it is most important to name the lives that have not mattered, and are struggling to matter in the way they deserve," Butler said in an interview with The New York Times . "If we jump too quickly to the universal formulation, 'all lives matter,' then we miss the fact that black people have not yet been included in the idea of 'all lives.'"