By Sebastien Malo

NEW YORK (Reuters) - Protesters in several U.S. 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.

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, some of whom appeared to be school-aged, 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.

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.

"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 the age of 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.

(Editing by Ellen Wulfhorst, Mohammad Zargham, Eric Walsh, Barbara Goldberg, Clarence Fernandez and Crispian Balmer Additional reporting by Eric M. Johnson in Seattle, Emmett Berg and Curtis Skinner in San Francisco and Mary Reardon in Madison)