William Cummings

USA TODAY

Demonstrators hurled rocks, bottles and at least one Molotov cocktail at police during a May Day march in Seattle that left five officers injured.

Police say nine people were arrested, including one juvenile female. Five of them were arrested for obstruction, three for assault, and one for destruction of property.

Of the injured officers, one was struck by a rock in the head, one was injured, but not burned, by a Molotov cocktail, and one was bitten.

Seattle Police Chief Kathleen O'Toole said the police department "has zero tolerance for any acts of violence towards persons, police and property."

O'Toole also said the department would review this year's May Day events to improve its crowd management techniques.

“While we hope for peaceful demonstrations, we need to be prepared for the unexpected," said O'Toole. "Officers underwent enhanced crowd management training and developed a comprehensive preparation plan for May Day, and the numerous protests that take place in our city year round."

Police used pepper spray on the demonstrators and at one point fired rubber bullets at the crowd. People then threw rocks and bottles at police.

A march in support of workers and immigrants' rights earlier in the day was without incident. The march was just one of several events in cities nationwide Sunday to call for better wages for workers and an end to deportations.

Around the world, union members have traditionally marched on May 1 for workers' rights. In the United States, the annual events have become a rallying point for immigrants and their supporters since massive demonstrations in 2006 against a proposed immigration enforcement bill.

Seattle is often home to large and raucous May Day celebrations. The nine arrests this year were less than in the previous three years.

Contributing: KING-TV, Seattle; The Associated Press