Swarms of police on bikes and in riot gear tried to corral the estimated 75 early-afternoon demonstrators with batons and chemical spray, but the protesters quickly dispersed, The Seattle Times reported.
Some reintegrated into the crowd, shedding their black clothes, Mayor Mike McGinn said at a hastily called news conference.
The vandals had smashed bank and car windows, spray-painted graffiti on financial-services offices and expensive cars, slashed tires, vandalized retail stores, set off sound bombs and fireworks, shot paint balls and set a small fire at a courthouse.
Police made eight arrests.
McGinn issued an emergency order after the 30 minutes of violence, authorizing police to confiscate sticks, tire irons, hammers and other equipment that could be used for destruction.
He also authorized police to pre-emptively seize the 5-foot-long poles the intruding demonstrators used.
"The First Amendment uses of 5-foot-long, 3-inch-wide sticks is outweighed by our authority ... to confiscate weapons," he said.
In other Occupy movement marches, about 2,000 people paraded through the streets of downtown Portland, Ore., Tuesday afternoon and evening, chanting for jobs and justice and against the jailing of immigrants.
Police took at least nine people into custody.
Thousands of protesters took to the streets of Oakland, Calif., with police firing tear gas and flash-bang grenades at demonstrators who clashed with police. At least nine people were arrested.
About 5,000 people marched to Oakland's Frank H. Ogawa Plaza, where violent and destructive riots broke out last fall when police tried to clear out the Occupy encampment from the 160,000-square-foot public space near City Hall.
The march came the same day a U.S. district court gave Oakland less than a week to submit a plan for completing police internal-affairs investigations stemming from last year's protests.
Judge Thelton Henderson issued his order a day after a federal monitor criticized Oakland police for using "an overwhelming military-type response" to the demonstrations.
In Los Angeles, several thousand people marched through downtown, with some briefly clashing with police in riot gear. A helmeted female police officer was struck in the head with a skateboard and taken to a hospital, police said.
There were no immediate reports of arrests.
Occupy protests -- which organizers called a "general strike," or "a day without the 99 percent" -- were held in more than 130 other U.S. cities and about 80 countries, where demonstrators demanded new jobs, better education, halts to austerity programs and initiation of increased government spending, ends to political corruption and the creation of radical political reforms, among other changes.
Large crowds marched in the Philippines, Tunisia and Spain.
In New York, the birthplace of last year's nationwide Occupy movement, the campaign held a series of May Day protests, the largest drawing thousands to Union Square in Lower Manhattan.
There were occasionally bloody clashes, The New York Times reported, citing a man with a bloody face led away in handcuffs.
More than 30 demonstrators were arrested, all on disorderly conduct charges, with most arrested for blocking traffic or resisting arrest, police said.
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