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Protesters outside the Republican National Convention burned American flags yesterday, leading to an extraordinary moment in which an officer shouted at a man who was about to go up in flames himself.

"You're on fire! You're on fire, stupid!" the Cleveland officer yelled at the protester, whose clothes appeared to be catching on fire.

Officers made 18 arrests, including this female flag-burner who was caught on tape.

She reportedly yelled "America was never great" as she was taken into a police van.

Police said two officers were hurt in the skirmishes, but not seriously.

One of the flag-burnings was staged by demonstrators from the Revolutionary Communist Party in response to the "crimes of the American empire."

It was the most confrontational moment of the week among protesters, who have been otherwise peaceful, and Cleveland police.

Officers will be on high alert tonight as Donald Trump makes his much-anticipated speech to the convention.

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