About 250 activists marched across New York's Brooklyn Bridge, holding up signs that read "Stop murder by police" and "Stop killer cops".

At least 12 people were arrested following a brief scuffle with police after they crossed the bridge, and long traffic delays were reported.

The demonstration was organized by the Stop Mass Incarceration Network following the April 4 fatal shooting of Walter Scott, an unarmed black man shot in the back by a white police officer in North Charleston, South Carolina.

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fatal shooting of Walter Scott

The killing -- just one of a succession of fatal police shootings -- was captured on video, and the officer has been charged with murder.

Police in Los Angeles said they arrested 15 protesters from a group of nearly 100 after they stopped on Metro Rail tracks and ignored orders to disperse.

Elsewhere on the West Coast, more than 100 protesters surrounded a police station in San Francisco and disrupted a meeting at City Hall.

In nearby Oakland, demonstrators massed outside the Oakland Police Department and poured onto Interstate 880, television broadcasts showed.

Rush hour on the Bay Bridge linking San Francisco to Oakland was briefly delayed when several protesters tried to block traffic, police said. Six demonstrators were arrested.

In Wisconsin, about 100 protesters, mostly high school students, blocked a major roadway in Madison, where last month's fatal shooting of unarmed black teen Tony Robinson Jr. by a white police officer has triggered a series of demonstrations.

New York police said an off-duty officer who was not in uniform was left with bruises on his head and arm after being struck by a protester on the Brooklyn Bridge when he exited his stopped car during the demonstration.

Protesters said they hoped their march would galvanize debate about the use of deadly force by police against minorities, with the families of several unarmed black or Hispanic men or boys who died in encounters with police demanding more oversight.