Nineteen Bay Area demonstrators were arrested Tuesday night in what was the fourth night of protests over police killings of unarmed black men.
Hundreds of protesters marched through Berkeley, Emeryville and Oakland, according to the California Highway Patrol.
But after some refused to get off a freeway, the CHP said, officers used less than lethal force to subdue the protesters and make arrests.
"Our fear, and what we are trying to prevent, is someone getting seriously injured or killed by choosing to march onto the freeway," Assistant CHP Chief Ernie Sanchez said in a statement.
About 9:17 p.m. a large group demonstrators walked onto the 24 Freeway at 40th Street, causing drivers to swerve to avoid hitting them.
Officers were able to clear the road, but said the protest turned violent, with high-powered fireworks launched at a CHP helicopter.
Protesters threw projectiles, incendiary devices and rocks at officers, the CHP said.
After giving an order to disperse, the CHP said several aggressive protesters remained on the freeway.
The CHP arrested 13 people for violations including creating a public nuisance, obstructing free movement of others and battery on a peace officer. In Berkeley, police arrested six protesters.
Earlier in the day Berkeley Mayor Tom Bates canceled Tuesday night's City Council meeting, expecting a crowd that would overwhelm the chambers.
On Monday, about 1,500 protesters marched throughout the region.
KTVU reported that a man was punched during protests in Oakland as he tried to put out a fire.
The Society of Professional Journalists' Northern California Chapter reported that several news reporters covering Saturday's protests were assaulted by police.
"It is important for journalists to be present at these events," the group's President Lila LaHood said.
"Journalists cover public protests for no reason other than to inform the public. Their mistreatment and harassment by law enforcement is a serious attack on the public's right to know and should not be tolerated."
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