By Edward Upright and Zachary Faginson
NEW YORK/MIAMI (Reuters) - Trayvon Martin's mother choked back tears as a crowd of 2,000 New Yorkers chanted "We love you" - one week after George Zimmerman was acquitted of murder by a Florida jury in the fatal shooting of the unarmed black teenager.
Across the nation, hundreds marched in the heat of a summer Saturday to rally at federal courthouses in Miami, Chicago, Los Angeles and other cities, demanding "Justice for Trayvon."
In Miami, Tracy Martin told about 300 supporters of his son's cause that, after the acquittal, he has "come to realize George Zimmerman wasn't on trial - Trayvon was on trial."
In New York, hip-hop mogul Jay-Z and singer Beyonce, his wife, arrived at one of the largest of the protests organizer Rev. Al Sharpton said were planned for 100 cities nationwide.
Martin's mother stifled sobs as she told the crowd: "Not only I vow to do what I have to do for Trayvon Martin, I promise I'm going to work hard for your children as well."
Among "Boycott Florida" signs were protesters wearing T-shirts with a photo image of Martin in a hooded sweatshirt.
"I've got four beautiful daughters. I want them to look forward rather than behind their backs," said Harlem resident Maria Lopez, 31, who attended the rally with her children.
Visible above the neckline of her Trayvon T-shirt was a tattoo of a packet of Skittles, the candy the teen was carrying when he was shot dead by the neighborhood watchman.
Civil rights leaders had voiced hopes for peaceful protests after outbreaks of violence that earlier this week led to arrests in Los Angeles and the San Francisco Bay area.
Late last Saturday night, a Seminole County jury in central Florida acquitted George Zimmerman, who is part Hispanic, of second-degree murder and manslaughter in the incident, where 17-year-old Martin was shot through the heart.
About 500 people converged on the federal courthouse in Los Angeles under gray skies, toting signs saying ‘Open Season on the Black Man' and ‘This Should Not Be OK in 2013 America."
Protesters' chants - "No Justice, No peace" - echoed across the courthouse plaza in call-and-response form.
Another speaker shouted out: "Who was that cryin'?" and the crowd responded: "Trayvon Martin." That exchange was in reference to conflicting testimony about the high-pitched screams for help captured on the 911 call, that were identified by Martin's mother as being Trayvon's, and by Zimmerman's mother as her own son's.
Farther north in foggy downtown San Francisco, about 100 people stood in front of the Federal Building.
Reverend Arnold Townsend, 70, vice president of the local NAACP chapter, vowed to "bring to light this incident let black children know the system has them under attack."
At the White House on Friday, President Barack Obama cautioned against violence, as he urged all Americans to try to understand the Martin case from the perspective of African-Americans.
"There is a history of racial disparities in the application of our criminal laws," Obama said. "A lot of African-American boys are painted with a broad brush. If a white male teen was involved in the same kind of scenario ... both the outcome and the aftermath might have been different."
Zimmerman remained free for more than six weeks after the incident because Sanford, Florida, police accepted that he had acted in self-defense. That ignited protests and cries of injustice across the country, shining a spotlight on issues such as race, profiling and vigilantism.
'A WORLD WHERE RACISM EXISTS'
Sharpton has said he hopes continued public pressure will force the Justice Department to bring a civil rights case against Zimmerman.
Federal prosecutors have said they are investigating whether Zimmerman violated civil rights laws. But lawyers with civil rights expertise have said they think new charges are unlikely.
Public comments from one of the six jurors, citing Florida's Stand Your Ground law as a factor in reaching her conclusion that Zimmerman acted in self-defense, has stepped up pressure on the state's legislature to repeal or change the law.
The jury was told that Zimmerman had "no duty to retreat and right to stand his ground and meet force with force, including deadly force, if he reasonably believed it was necessary to do so to prevent death or great bodily harm to himself."
Although Stand Your Ground was not cited as part of the defense, the jury's instructions came from the 2005 statute.
Florida Governor Rick Scott told a sit-in outside his office in Tallahassee on Thursday that he supports the law and has no plans to convene a special legislative session to change it.
"We still live in a world where racism exists,'' said Rev. Reginald Edwards at the U.S. District Courthouse in Tallahassee, where nearly 80 protesters assembled at midday to urge federal officials to charge Zimmerman with civil rights violations.
(Additional reporting by Tom Brown in Miami,; Emmett Berg in San Francisco and Dana Feldman in Los Angeles, Bill Cotterell in Tallahassee, Fla.; Writing by Chris Francescani; Editing by Dina Kyriakidou, Barbara Goldberg and Gunna Dickson)