The angry protests highlight simmering racial tensions after a jury found volunteer watchman George Zimmerman not guilty of murdering 17 year-old Trayvon Martin.

Demonstrators held noisy rallies in US cities including New York, San Francisco, Boston, Detroit and Chicago.

At least six people were arrested in Los Angeles early Monday when riot police broke up an "unlawful assembly" in Hollywood near the CNN building.

To the chants of "No justice, no peace!" a separate crowd blocked traffic on an important freeway elsewhere in the giant metropolis, local media reported.

Memories are still fresh of the deadly April 1992 riots in Los Angeles, which broke out after a similar racially-tinged trial.

The most numerous rally Sunday was in New York City, where several thousand -- including families with children -- marched to Times Square under the watchful eye of police.

Many in the multi-racial crowd brandished signs bearing a portrait of Martin, while some, despite sweltering July heat, wore "hoodie" sweatshirts as the teen did the night he was killed.

"I am appalled," said Carli VanVoorhis, a 21-year-old hairdresser.

"The man was armed, the kid was not, and the man with the gun got away," she said. "If we say it was not a racial issue, we would be lying."

"We have a big problem with race, and another problem is guns," said protester Rodney Rodriguez. "If Zimmerman didn't have a gun, he couldn't have killed Trayvon Martin."

Fellow protester Derreck Wilson, 46, said the group had come "to say in a peaceful way why we are angry. We are angry, scared and anxious."

"It's cathartic," he said.

"We all have the same desires. I want to be able to have my son to come home," added Wilson, who came to the protest from the traditionally African-American neighborhood of Harlem.

The case has pitted those who believe that Zimmerman, a 29-year-old Hispanic neighborhood watchman, killed Martin in self-defense, and those who believe it was a murder sparked by racist assumptions.

Obama, the first black US president, urged Americans to step back and accept the trial verdict.

"We are a nation of laws, and a jury has spoken," the president said in a statement. "I now ask every American to respect the call for calm reflection from two parents who lost their young son."

Zimmerman was accused of pursuing Martin through a gated community in the town of Sanford and shooting him during an altercation on the rainy night of February 26, 2012.

The defense successfully argued that Zimmerman fired his handgun in self-defense after the teen wrestled him to the ground and was slamming his head against the pavement.

According to Florida's controversial "Stand Your Ground" law, people who fear for their lives can use deadly force to defend themselves without having to flee a confrontation.

"We should ask ourselves, as individuals and as a society, how we can prevent future tragedies like this," Obama said. "As citizens, that's a job for all of us. That's the way to honor Trayvon Martin."

Obama last year spoke emotionally about the case, noting that if he had a son he would "look like Trayvon."

The racial divide was evident in Sanford pastor Valerie Houston's Sunday sermon.

"Dr (Martin Luther) King (Jr) stated, the daily life of the Negro is still in the basement of the Great Society," she said. "And today I state, the daily life of my people is still enslaved to a white supremacist society."

Martin's parents -- father, Tracy, and mother Sybrina Fulton -- asked the public before the verdict to respect the trial outcome, and afterwards gave thanks for the outpouring of support they received over the past year.

Martin family attorney Benjamin Crump declined to say whether they would file a civil lawsuit against Zimmerman, but said "they are going to certainly look at that as an option."

"They deeply want a sense of justice. They deeply don't want their son's death to be in vain," he told ABC News's "This Week."

The US Justice Department said Sunday it will resume its probe into whether federal charges can be brought against Zimmerman. The probe was put on hold until after the Florida trial.

Federal prosecutors "will determine whether the evidence reveals a prosecutable violation of any of the limited federal criminal civil rights statutes within our jurisdiction," it said in a statement.

With the acquital, Zimmerman's lawyers told the Washington Post that they will immediately resume a defamation lawsuit against NBC News for selectively editing a phone call to police which made their client seem like a racist. That case was also put on hold pending the trial results.