By late evening Wednesday, police said 18 people had been arrested for assault and property damage, the Seattle Post-Intelligencer reported.
Eight police officers were injured in the confrontations.
Using Twitter, police reported groups of protesters gathering in several locations throughout Seattle, including Seattle Central Community College. The Twitter feeds said protesters seemed to be lighting "flares or smoke devices" at one location and that windows were being damaged at another.
Police later reported on Twitter objects were thrown at them and that they had used pepper spray "to clear the streets."
Police tried to disperse protesters with flash-bang devices and pepper spray and reported bottles and other objects were being thrown at them, the Post-Intelligencer said.
A separate, early afternoon May Day march in support of immigration reform drew the largest crowd, estimated to be 3,000-4,000 people, The Seattle Times reported.
Marchers chanted and waved signs as they marched from Judkins Park through the Chinatown International District to the Henry M. Jackson Federal Building in downtown Seattle.
This year's march occurred two weeks after a bipartisan group of U.S. senators introduced a bill that, among other things, would grant legal status to an estimated 11 million people in the United States illegally.
"This is it," marcher Mauricio Ayon, legislative director for Washington Community Action Network, told the Times. "The tide is turning on this issu, and I don't think anybody wants to be caught on the wrong side of it."