Protesters repeatedly interrupted the Democrats' presidential primary debate Wednesday night in Detroit -- and at one point, demonstrators shouted down frontrunner Joe Biden with chants of "three million deportations."

That number referred to the approximate number of illegal immigrant deportations under the Obama administration -- which, on matters from health care to immigration policy, came under fire from the 2020 hopefuls as the debate continued.

Biden did not directly respond to the demonstrations. He did say at one point during the debate, to the ire of some progressive commentators: "If you cross the border illegally, you should be able to be sent back. If we let people in, what should we say to the other immigrants wanting to come to the U.S. around the world? … People should have to get in line."

In July, Biden similarly refused to apologize for the nearly three million deportations on his watch, after being confronted by protesters while campaigning in Dover, New Hampshire.

Earlier in the debate, a separate group of protesters erupted in chants of "Fire Pantaleo," in reference to the 2014 death of Eric Garner, as New York City Mayor Bill de Blasio delivered his opening statement.

New York City Police Officer Daniel Pantaleo was the officer who put Garner, an unarmed black man, in a chokehold after the 43-year-old was attempting to sell cigarettes on the streets. The incident was caught on camera, where Garner could be heard saying "I can't breathe" several times before he died.

De Blasio later was asked about the Pantaleo case during the debate, but did not explain why the officer has not yet been fired.

The Pantaleo protests continued as Sen. Cory Booker, D-N.J. was making his introduction, which halted the entire debate.

De Blasio's team tweeted as the debate continued: "To the protestors in the audience today: I heard you. I saw you. I thank you. This is what democracy looks like and no one said it was pretty."

The tweet continued: "I want the Garner family and every single person hurt by the tragedy of his death to know they are seen and heard. We all watched Eric Garner's dying words. They haunted this nation. He NEVER should have died. While I believe that respecting the process is the best way to get justice for Eric Garner's family, I recognize and identify with the pain people across this country are feeling.

"From ending a broken policy of stop-and-frisk to training our officers in implicit bias, we've fundamentally changed our city because of Eric Garner — so that a tragedy like this never happens again," de Blasio's team concluded.

After the outburst that unfolded while Booker spoke, Fox News learned Detroit police removed five demonstrators. Among them was Linda Sarsour, an activist who has been called anti-Semitic, including for supporting the Boycott, Divestment, Sanctions (BDS) movement against Israel.

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All five told Fox News they came to protest de Blasio's handling of the Garner case, and Pantaleo.

Meanwhile, the Twitter account of Democratic presidential candidate Mike Gravel -- which is normally run by teenagers -- issued a bounty for more interruptions.

"If you're in the audience tonight, interrupt Joe Biden!" the account said. "Take a video, send it to us, and we'll give you $100."

Gravel did not qualify to appear at the debate Wednesday, according to the DNC.

Fox News' Andrew Keiper in Detroit, Patrick Ward, Joseph Wulfsohn and Nick Givas contributed to this report.