A day of peaceful May Day demonstrations in Seattle was followed by violence on Friday, CNN reports, with black bloc protesters hurling bricks, wrenches and burning objects at police and officers responding with pepper spray and flash bang grenades.

“During this moment in history, peaceful protest and civil disobedience can be effective vehicles of social change,” said Mayor Ed Murray in a statement. “What erupted tonight is a very different story. Tonight we saw assaults on police officers and senseless property damage, which cannot be tolerated. Those who are violent will be arrested.”

According to police, 16 people were arrested at the “unplanned and un-permitted” demonstration on charges ranging from failure to disperse to assault. From Reuters:

Footage on social media showed protesters smashing shop windows in Seattle and crowds scattering as police in riot gear threw in “flashbang” grenades. Demonstrators set fire to garbage and damaged at least two dozen vehicles, police said.

Three officers were reportedly injured in the clash, suffering from a dislocated shoulder, a broken wrist and leg burns. According to The Seattle Times, several protesters say they suffered injuries from pepper spray and police projectiles.

[Image via AP Images]