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It happened during a protest over racism and police violence after the death of George Floyd.
"You've communicated a sense of urgency," former US President Barack Obama told young protesters, calling protests over the death of George Floyd "powerful".
"Donald Trump is the first president in my lifetime who does not try to unite the American people —does not even pretend to try. Instead, he tries to divide us," said Mattis, who served as Trump's defence secretary from 2017-2019.
COVID-19 cases in the US pass the one million mark and more Americans have lost their lives to the disease than died during the Vietnam War.