(Reuters) - Seattle police said on Saturday they were preparing for more weekend protests after a May Day march that turned violent when crowds hurled bottles and wrenches at officers, who responded with pepper spray and flash grenades.

Sixteen people were arrested in the violence, which erupted late Friday after a day of peaceful demonstrations to call attention to workers' rights, immigration issues and police treatment of minorities in the United States.

Three officers were hurt, including one who suffered a broken wrist and another with a dislocated shoulder, police said.

Law enforcement was prepared for more weekend protests, department spokesman Drew Fowler said.

"We are prepared to support all demonstrations of free speech," he said in an email. "However if there is physical violence or property destruction we will take action to protect public safety."

Protesters annually assemble on May 1 as a day to focus attention on labor and immigration issues, but demonstrators in cities across the country also used the occasion to rally against police violence.

The violent protests in Seattle contrasted with peaceful and even celebratory demonstrations in Baltimore after prosecutors brought charges on Friday against all six officers involved in the arrest of Freddie Gray, the 25-year-old black man who died of spinal injuries suffered while in police custody last month. On Monday, the city was rocked by looting, arson and clashes with police.

In Seattle, anti-capitalist protesters wearing masks hurled wrenches, bottles and other items late Friday at officers.

The violence was echoed in other U.S. West Coast cities, with peaceful May Day marches breaking off into unsanctioned gatherings in Portland and Oakland, California.

A dozen people were arrested in Oakland after some demonstrators threw rocks and other items at cars and businesses, authorities said.

"Vandalism included broken windows, graffiti, small fires and failure to disperse," Oakland police said in a statement.

Pepper spray and flashbangs were also used in Portland after some protesters threw objects at officers and tried to force their way onto a bridge, the city police department said on its Twitter account. One officer was injured, it said.

Seattle police said they would review the actions used by officers in accordance with a federal monitoring agreement to stem a pattern of excessive force in the department, but added early indications showed that law enforcement used restraint and acted appropriately.

(Editing by Frank McGurty and Clelia Oziel)