A protester interrupts O’Malley, right, as moderator Jose Vargas looks on at Saturday’s Netroots Nation event in Phoenix. (Photo: Ross D. Franklin/AP)

Former Maryland Gov. Martin O’Malley apologized Saturday for saying “all lives matter” after he was interrupted by protesters chanting “black lives matter” at a Netroots Nation conference, a gathering of liberal activists, in Phoenix.

“That was a mistake on my part, and I meant no disrespect,” O’Malley said in an online interview after the event. “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.”

O’Malley and Vermont Sen. Bernie Sanders were taking part in a presidential town hall when several dozen protesters interrupted the session, demanding the Democratic candidates address recent cases of brutality against African-Americans by white police officers.

“Black lives matter, white lives matter,” O’Malley responded. “All lives matter.”

This crowd does not want to hear “white lives matter.” — Jane C. Timm (@janestreet) July 18, 2015

O'Malley just said #AllLivesMatter which means he just doesn’t get it. #NN15 — Emily Epstein White (@EmEps) July 18, 2015

Did he just say all lives matter? Did no one prep O'Malley for this room? #NN15 — Jane Booth-Tobin (@jboothto) July 18, 2015

The “black lives matter” movement began in 2013 in protests following the acquittal of George Zimmerman in the shooting death of Trayvon Martin, an unarmed black teenager, in Sanford, Fla., and became a rallying cry for demonstrators in Ferguson, Mo., and Staten Island, N.Y., in 2014 and Baltimore earlier this year. “All lives matter” has been used by some in response, angering those who say such a reply misses the point.

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“When some people rejoin with ‘all lives matter,’ they misunderstand the problem, but not because their message is untrue,” Judith Butler, a comparative literature professor at the University of California, Berkeley, told the New York Times earlier this year. “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 added, “Claiming that ‘all lives matter’ does not immediately mark or enable black lives, only because they have not been fully recognized as having lives that matter.”

O’Malley isn’t the first Democratic hopeful to come under fire for using the “all lives matter” phrase. Last month, Hillary Clinton was criticized for using it at a historic black church in Florissant, Mo., less than 5 miles away from Ferguson:

Before using the phrase, Clinton was retelling an anecdote about the lessons she learned from her mother. “I asked her, ‘What kept you going?’ Her answer was very simple. Kindness along the way from someone who believed she mattered. All lives matter.”

“With her statement that all lives matter, that blew a lot of support that she may have been able to engender here,” Renita Lamkin, a pastor at the St. John AME Church in St. Charles, Mo., told NPR. “My children matter. And I need to hear my president say that the lives of my children matter. That my little black children matter. Because right now our society does not say that they matter. Black lives matter. That’s what she needs to say.”

While O’Malley faced the brunt of the backlash, Sanders did not escape entirely unscathed. When the Vermont independent senator tried to speak, he was shouted down, too.

“Black lives, of course, matter. I spent 50 years of my life fighting for civil rights and for dignity,” Sanders said. “But if you don’t want me to be here, that’s OK. I don’t want to outscream people.”

But when Sanders’ supporters suggested the demonstrators ought to support him, noting the senator was among the 200,000 people in attendance for the Rev. Martin Luther King’s “I Have a Dream” speech, they faced their own backlash.

A Twitter hashtag mocking Sanders, #BernieSoBlack, quickly began trending and soon took on a life of its own.

Hold up! Bernie Sanders marched with MLK? This changes EVERYTHING! — Rod TBGWT (@rodimusprime) July 19, 2015

I actually heard it was Bernie’s idea to march in Selma. MLK wanted to do the march in Hawaii. A destination march. #BernieSoBlack — Rod TBGWT (@rodimusprime) July 19, 2015

I heard Bernie Sanders told Joe - you and Katherine got so many kids you should put a band together, lil Michael got talent #BernieSoBlack — el DP (@the_otherDP) July 19, 2015

#BernieSoBlack he dies first in every horror movie — Tim Black (@TimBartender) July 19, 2015

#BernieSoBlack Rachel Dolezal claimed he’s her grandfather. — Danielle (@abradacabla) July 19, 2015