Hundreds of peaceful demonstrators took part in a sunny May Day march in support of immigrant rights and a boost in the minimum wage in Seattle, but a smaller group of sometimes-masked protesters kept police busy as the sun went down.
Police say they arrested nine people during the Thursday evening marches.
Officers say they used pepper spray after some of the night marchers threw bottles at police in downtown Seattle. Protesters later started trash can fires in the Capitol Hill neighborhood. One arrest came after a brick was hurled at police.
Police accompanied the anti-capitalist protesters as they marched in a rambling circuit from Capitol Hill to downtown Seattle and back again before officers hemmed them in.
Violence has plagued May Day in Seattle during the past two years, with protesters challenging police in the streets.