Cleveland (CNN) Cleveland police on Wednesday made 18 arrests and reported that two officers had been assaulted after a protester attempted to light an American flag on fire outside the security barrier surrounding the Republican National Convention, authorities said.

Upward of 200 law enforcement officers from dozens of departments, joined by Cleveland Police Chief Calvin Williams, converged on the scene, confronting a small group of demonstrators in what quickly escalated into the most heated clash in a mostly subdued week of protests.

"You're on fire! You're on fire, stupid," an officer could be heard yelling when the man who tried to start the blaze had his leg briefly catch fire. He was then arrested for assaulting an officer, Williams said during an evening news conference, after trying to bat away police seeking to douse the flames.

Hours after dispersing the crowd, Cleveland police tweeted that the officers had been punched and pushed during the brief melee. Their injuries were described as minor. Charges against those arrested included felonious assault on a police officer, failure to disperse and resisting arrest.

In a joint statement released Wednesday night, Williams and Cleveland Mayor Frank Jackson said 18 arrests had been made over the course of the day.

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