Protesters described as "hostile" and "aggressive" over the killing of unarmed black men by white police officers briefly blocked a highway in Berkeley, Calif., nearly preventing medics from reaching a pregnant woman stuck in traffic, set garbage cans on fire, threw rocks at police and brought mass transit to a halt during the city's fourth night of protests.

"The crowds have become larger, more hostile and more aggressive," Eric Sanchez, an assistant division chief for California Highway Patrol, told the San Francisco Chronicle. The protests started peacefully, but just before midnight small groups broke off and began breaking store windows, including a 7-Eleven and Safeway.

The protests started at about 5 p.m. near the UC Berkeley campus, with about 100 people. The number grew to 300 by 8 p.m., The Los Angeles Times reported.

A City Council meeting scheduled for Tuesday night was canceled after threats to disrupt it, said Berkeley Mayor Tom Bates. Protesters were reportedly unhappy about the decision.

A California Highway Patrol official said 80 percent of its available staff would be deployed to monitor the protest in Berkeley after a crowd of about 1,500 blocked all lanes of Interstate 80 and blocked an Amtrak train Monday night. Protesters on Tuesday night breached a fence and briefly blocked the westbound lane on Highway 24. Several arrests were made after cars reportedly screeched to a halt as police cleared out protesters.

A large group of demonstrators destroyed highway perimeter fencing, flooded lanes, and threw rocks and other objects at officers. It took about 90 minutes to clear the interstate, and no major injuries were reported, authorities said.

A woman stuck in traffic went into labor during the protest, but fire crews were able to get her to a hospital, KPIX-TV reported.

The Chronicle reported that many of the protesters yelled with their hands raised, "Hands up, don't shoot!" Some threw fireworks. They were eventually met by residents and peaceful protesters who tried to stop them, the report said.

The newspaper spoke to one resident named "Willie" who retrieved a large trash can that was lit on fire.

"This is the Dumpster for my building," he said. "We need this Dumpster."

Sanchez told the San Francisco Chronicle the agency also will ask the Alameda County district attorney's office to increase bails and charges for those arrested. Those arrested face bails of up to $50,000, and many remain in custody, he added.

"The CHP respects the public's right to gather and demonstrate, but it needs to be done in a safe manner," Sanchez said. "At this point, they've made their statement and we respect that. Now, we're asking them to stop."

Transit officials said the station in downtown Berkeley was closed as a precaution. Amtrak train service was suspended between the Oakland Coliseum station stop and Richmond because of the protest, officials said.

Although many activists in other parts of the country have gone home, protests in Berkeley and Oakland are still active, reflecting the area's long history of protest dating to the 1960s.

The protests started after a grand jury on Nov. 24 declined to indict Officer Darren Wilson in the killing of Michael Brown in Ferguson, Mo. A New York grand jury on Dec. 3 declined to prosecute a police officer captured on video applying a chokehold on Eric Garner moments before he died. That decision set off more demonstrations nationwide.