Three people were arrested Tuesday for burning an American flag during a May Day protest in an uptown park in Charlotte, North Carolina, police said.

Charlotte-Mecklenburg Police told the Charlotte Observer that while 15 protesters gathered in the park for the Tuesday evening event, three people took an American flag, doused it in “ignitable liquid,” and set it ablaze at around 6:20 p.m.

The event, which took place at Marshall Park, was to protest International Workers’ Day—also known as May Day.

Police said they permitted the protesters to demonstrate at the park and were present at the event to ensure the protesters’ safety during their march to College and Trade Streets, WCCB reported.

When the protesters made it to North College Street, three people allegedly stopped to set an American flag on fire.

Charlotte Fire and arson detectives with the police department responded to the scene and arrested Dhruv Pathak, 24; Landon Rice, 23; and VanaMary Isaac, 26. Each of them was charged with injury to real property and carelessness with fire in connection with the alleged flag-burning incident.

The Supreme Court ruled that flag burning is constitutionally protected speech under the First Amendment of the Constitution and is legal. Police said, however, that setting items on fire in a city park is illegal in Charlotte and is the reason they arrested the protesters.

Although the Supreme Court ruled flag burning as a form of free speech, some have shared displeasure with how some people treat the American flag.

Shortly after Donald Trump won the presidential election in November 2016, he suggested that people who burn the American flag should spend a year in jail or lose their citizenship.