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Eric Garner, Michael Brown Protesters in Berkeley Stop Amtrak Train

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Demonstrators upset by the police killings of unarmed black men rallied again nationwide Monday, stopping an Amtrak train in California and holding "die-ins" outside the Brooklyn Nets basketball arena as Britain's Prince William and his wife Kate attended a game.

In Berkeley, California, demonstrators blocked traffic on Interstate 80, forced a train to stop and briefly shut down a subway station. Marches turned violent there over the weekend, with protesters throwing rocks at police and vandalizing California Highway Patrol vehicles. Authorities deployed tear gas.

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.

Berkeley Mayor Tom Bates said a tiny fraction of protesters are obscuring the wider message calling for reform of policing policies nationwide. Bates called the violent elements among the demonstrators "cowards and thugs who need to take off their masks."

The protests started after a Nov. 24 decision by the grand jury not to indict officer Darren Wilson in the killing of Michael Brown in Ferguson, Missouri. A New York grand jury on Dec. 3 declined to prosecute a police officer captured on video applying a fatal chokehold on Eric Garner.

Protesters also took to the pavement in Philadelphia, where a die-in — in which demonstrators lie down on the street — was held along the city’s famed “Main Line.”

In New York, city council members, clergy and protesters staged a "die-in'' on the steps of City Hall. Later Monday, a few hundred people gathered outside Barclays Center. "If the royal family is here, we want to let them know what has happened in our city," Carmen Perez, a co-founder of Justice League NYC — a group that helped organized demonstrations — told The Associated Press.

Protests weren't the only responses to the grand jury decisions. In Chicago on Monday, four of the city’s alderman proposed changing the municipal code to ban the use of chokeholds by peace officers or private security personnel “unless faced with a situation in which the use of deadly force is justified under applicable law."

NBC News' Shamar Walters and The Associated Press contributed to this report.

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