In Los Angeles, several hundred mostly peaceful protesters gathered Monday night at Leimert Park southwest of downtown, many of them chanting, praying and singing.
But a smaller group of about 100 people splintered off and began blocking traffic on nearby Crenshaw Boulevard, some of them jumping on cars and breaking windows.
There was at least one arrest and more were expected, Los Angeles Police Lt.
Several protesters ran into a Wal-Mart store, where they knocked down displays before store security chased them out, and police began guarding the door.
TV news helicopters showed some people apparently throwing punches along the street. There were no immediate reports of injuries.
"I commend the prayer rally attendees in Leimert Park for practicing peace," tweeted LA Mayor Eric Garcetti, who returned early from an East Coast visit. "I call on people in street on Crenshaw to follow their example.
In Oakland, dozens of demonstrators briefly blocked all lanes of Interstate 880 at the tail end of rush hour Monday evening, stopping traffic in both directions for several minutes before they were cleared by authorities. Several protesters laid their bicycles on the ground in front of stopped cars.
"You've got to go. You will go to jail," one police officer shouted at demonstrators who were blocking traffic, the Oakland Tribune reported.
Later, another group tried to march up the onramp to Interstate 580 before being turned away by Oakland police and California Highway Patrol officers.
The freeway protesters broke off from a larger group organized via social media that gathered at Oakland City Hall about an hour earlier. Several people were arrested for acts of violence and vandalism while marching from City Hall, authorities said.
Over the weekend, demonstrators in Oakland and Los Angeles blocked traffic and clashed with police in protests over a Florida jury's acquittal of neighborhood watch volunteer Zimmerman in the shooting death of the unarmed black teenager.
Police shot beanbag rounds and arrested six people—including one on suspicion of assaulting an officer—while breaking up relatively small demonstrations before dawn.
No injuries were reported to either demonstrators or officers.
Most demonstrations around the state were peaceful.
Democratic Sen. Barbara Boxer sent a letter Monday to U.S. Attorney General Eric Holder supporting the Justice Department decision to review the case to determine whether Martin's civil rights were violated.
"I respect the fact that the jury has spoken ... but I don't think this should be the last word," Boxer wrote in the letter.
"Trayvon Martin's death was a tragedy and has raised many sensitive and important issues," she wrote. "We should explore every avenue in an effort to ensure that something like this never happens again."