At a campaign rally for Bernie Sanders in Arizona on Thursday night, a protester unfurled a Nazi flag with a swastika behind the senator, who could become the first Jewish president in U.S. history. The Phoenix crowd's cheers for the Democratic candidate suddenly turned into intense boos when they noticed the man in the upper deck holding a red flag emblazoned with a swastika.

Security quickly removed the man from the Arizona Veterans Memorial Coliseum, and the flag was ripped from his hands by Sanders supporters. Sanders did not appear to turn around and see the flag, but he did comment on the disruption as the protester was escorted out.

"Whoever it was, I think they're a little outnumbered tonight," Sanders told the crowd, smiling.

A man brought a literal Nazi flag to the rally of a Jewish Socialist candidate for President He was escorted out by security forcespic.twitter.com/KOLhyJJFxZ — Siddak Ahuja (@SiddakAhuja) March 6, 2020

A disturbing video of a man shouting racial slurs outside the rally also circulated on social media. The man, wearing a white T-shirt, appears to be the same man who had been holding the Nazi flag.

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Asked about the incident Friday, Sanders confirmed he did not actually see the swastika flag and only learned about it after he left the stage.

"I will simply say this, and I speak not only as a Jewish American, I think I can speak for the families of some 400,000 American troops who died fighting Nazism, fighting fascism. That is, it is horrific. It is beyond disgusting to see that in the United States of America there are people who would show the emblem of Hitler and Nazism," he said. Sanders also praised the police at the event for doing "an extraordinary job."

Sanders is the son of Polish immigrants and has previously discussed his family members who were murdered in the Holocaust. Ahead of the rally, he tweeted: "I would be very proud to be the first Jewish president. Together, we will counter the hatred and bigotry of the Trump administration."

Sanders' campaign did not immediately respond to CBS News' request for comment. "The senator is aware of the flag with the swastika on it and is disturbed by it," Mike Casca, a campaign spokesman, told The New York Times.

"Targeting a Jewish candidate with a Nazi flag represents a new level of depravity," tweeted Jonathan Greenblatt, CEO of the Anti-Defamation League. "There is no place for hate in politics. Disagree on issues, but all good people should flat out reject this kind of poison when it appears in the 2020 race."

Targeting a Jewish candidate with a Nazi flag represents a new level of depravity. There is no place for hate in politics. Disagree on issues, but all good people should flat out reject this kind of poison when it appears in the 2020 race. https://t.co/4RaBfYkiOS — Jonathan Greenblatt (@JGreenblattADL) March 6, 2020

Steven Slugocki, chairman of the Maricopa County Democratic Party in the Phoenix area, called the incident "absolutely abhorrent."

"We can argue about which candidate should get the Dem nomination, but antisemitic acts have no place in this world," he tweeted.

The incident, which has been widely shared on social media, added to concerns over candidates' safety on the campaign trail. Just two days earlier, animal rights protesters rushed the stage in Los Angeles as former Vice President Joe Biden prepared to speak. Biden's wife and campaign press secretary leapt into action along with security guards to drag the protesters away.

Some supporters think both Sanders and Biden should have Secret Service protection. The Secret Service released a statement Thursday saying no candidates have requested protection by the agency, but that it is "fully prepared" to step in if requested.

Cara Korte contributed reporting.