The former US President Bill Clinton spent more than 10 minutes facing down protesters at a campaign rally in Philadelphia
They were protesting a 1994 crime bill he approved while president, which led to a surge in the imprisonment of black people.
Bill Clinton angrily told Black Lives Matter protesters that they "are defending the people who kill the lives you say matter," prompting criticism from black voters whose support Hillary Clinton is counting on in her quest for the presidency.
The Democratic race has become increasingly heated in recent days as Hillary Clinton, stung by a string of losses in state nominating contests, has traded barbs with her rival for the nomination, Bernie Sanders, over who is more prepared for the White House.
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In Philadelphia, several protesters heckled the former president mid-speech and held signs, including one that read "CLINTON Crime Bill Destroyed Our Communities."
Video footage of Hillary Clinton defending the bill in 1994 by calling young people in gangs "super predators" who need to "be brought to heel" have been widely circulated during the campaign by activists in the Black Lives Matter protest movement.
Bill Clinton defended her 1994 remarks, which protesters say were racially insensitive.
"I don't know how you would characterise the gang leaders who got 13-year-old kids hopped on crack and sent them out on the street to murder other African-American children," he said, shaking his finger at a heckler as Clinton supporters cheered, according to video of the event. "Maybe you thought they were good citizens. She (Hillary Clinton) didn't."
"You are defending the people who kill the lives you say matter," he told a protester. "Tell the truth."
Hillary Clinton, who also has faced protesters upset by her remarks, has said she regrets using the term.
Bill Clinton said last year that he regrets signing the Violent Crime Control and Law Enforcement Act into law because it contributed to the country's high incarceration rate of black people for nonviolent crimes.
His remarks on Thursday prompted criticism online as some saw him criticising the Black Lives Matter movement, which was the product of anger over a string of encounters around the country in which police officers killed unarmed black people.
Johnetta Elzie, a prominent civil-rights activist, wrote online that Clinton "can't handle being confronted by his own record."
"This is like watching a robot malfunction," she wrote.
Earlier in Philadelphia, Sanders, a US senator from Vermont, doubled down on his criticism that Clinton is unqualified to be president as the two campaigns became increasingly testy less than two weeks before New York's election contest.
"Are you qualified to be president of the United States when you're raising millions of dollars from Wall Street, an entity whose greed, recklessness and illegal behaviour helped destroy our economy?" Sanders said at a news conference.
Clinton this week sharply questioned Sanders' credentials and ability to carry out a campaign pledge to break up the big banks.
