Protesters in several US cities blocked highways and swarmed police precincts, leading to at least two dozen arrests in demonstrations touched off by fresh cases of police violence against unarmed black men.

Marching across New York's Brooklyn Bridge, about 250 placard-bearing activists organised by the Stop Mass Incarceration Network protested against the latest incidents of violent police tactics used against minorities.

Holding up signs reading "Stop murder by police" and "Stop killer cops," they said they hoped to spur national discussion on the issue. At least 12 people, some of whom appeared to be school-aged, were arrested following a brief scuffle with police after they crossed the bridge.

Police in Los Angeles said they arrested 15 protesters of 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 swarmed 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, the California Highway Patrol said.

Sign-waving marchers briefly blocked commuter traffic at several points in downtown Seattle, but the demonstration was peaceful and there were no arrests, police and transit officials said.

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.

Galvanizing their cause was 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.

The shooting was captured on video, and the officer has been charged with murder.

The families of several unarmed black or Hispanic men or boys who died in encounters with police demanded more oversight.

"What this protest right here is about is that too many are being murdered," said Nicholas Heyward Sr., who has struggled for years to reopen the case of his son, shot dead at age 13 by a police officer 20 years ago while playing cops and robbers with a toy gun.

"Not only do I have to wait, but while I'm waiting, I am constantly seeing innocent victims gunned down on the street for no reason at all," he added.

Last year, protests were sparked by a string of high-profile cases of black men losing their lives at the hands of white police officers.

But the outbursts of anger following the deaths of Michael Brown in Ferguson, Missouri, and Eric Garner in New York slowed to a standstill over the winter.

Another group of protesters, led by Justice League NYC, has embarked on a 250-mile trek to Washington from New York City, and is due to reach the National Mall on April 21.