The Justice Department said it is looking into the case to determine whether federal prosecutors should file criminal civil rights charges now that Zimmerman has been acquitted in the state case.
In New York City, hundreds of protesters marched into Times Square on Sunday night, zigzagging through Manhattan's streets to avoid police lines. Sign-carrying marchers thronged the busy intersection, chanting "Justice for! Trayvon Martin!" as they made their way from Union Square, blocking traffic for more than an hour before moving on.
In San Francisco and Los Angeles where an earlier protest was dispersed with beanbag rounds police closed streets as protesters marched Sunday to condemn Zimmerman's acquittal.
Rand Powdrill, 41, of San Leandro, said he came to the San Francisco march with about 400 others to "protest the execution of an innocent black teenager."
"If our voices can't be heard, then this is just going to keep going on," he said.
Earlier, at Manhattan's Middle Collegiate Church, many congregants wore hooded sweatshirts which Martin wore the night he was shot in a show of solidarity. Hoodie-clad Jessica Nacinovich said she could only feel disappointment and sadness over the verdict.
"I'm sure jurors did what they felt was right in accordance with the law but maybe the law is wrong, maybe society is wrong; there's a lot that needs fixing," she said.
At a youth service in Sanford, Fla., where the trial was held, teens wearing shirts displaying Martin's picture wiped away tears during a sermon at the St. Paul Missionary Baptist Church.
About 200 people turned out for a rally and march in downtown Chicago, saying the verdict was symbolic of lingering racism in the United States. Maya Miller, 73, said the case reminded her of the 1955 slaying of Emmitt Till, a 14-year-old from Chicago who was murdered by a group of white men while visiting Mississippi. Till's killing galvanized the civil rights movement.
"Fifty-eight years and nothing's changed," Miller said.
Protesters also gathered in Miami, Philadelphia and Washington, D.C along with a host of other cities.
In Miami, more than 200 people gathered for a vigil. "You can't justify murder," read one poster.
Carol Reitner, 76, of Miami, said she heard about the vigil through an announcement at her church Sunday morning. "I was really devastated. It's really hard to believe that someone can take the life of someone else and walk out of court free," she said.
In Philadelphia, about 700 protesters marched from LOVE Park to the Liberty Bell, alternating between chanting Trayvon Martin's name and "No justice, no peace!"
"We hope this will begin a movement to end discrimination against young black men," said Johnathan Cooper, one of the protest's organizers. "And also to empower black people and get them involved in the system."
Earlier Sunday, hundreds gathered in Union Square in New York City to voice their passions over the verdict, hoisting placards with images of Martin.
Some tempered their anger, saying they didn't contest the jury's decision based on the legal issues involved.
But "while the verdict may be legal, a system that doesn't take into account what happened is a broken legal system," said Jennifer Lue, 24.
Civil rights leaders, including the Revs. Jesse Jackson and Al Sharpton, urged peace in the wake of the verdict. Jackson said the legal system "failed justice," but violence isn't the answer.
But not all the protesters heeded those calls.
In Oakland, Calif., some angry demonstrators broke windows, burned U.S. flags and started street fires. In Los Angeles, police said a crowd of about 100 protesters surrounded an officer and had to be dispersed by officers firing beanbag rounds.
Obama: Preventing tragedies honors Trayvon Martin
President Barack Obama said Sunday that he respected a Florida jury's decision to acquit George Zimmerman in the shooting death of Trayvon Martin, but called on the nation to reflect on gun violence.
The president said in a written statement that Martin's death was a "tragedy. Not just for his family, or any one community, but for America."
"I know this case has elicited strong passions," he said. "And in the wake of the verdict, I know those passions may be running even higher. But we are a nation of laws, and a jury has spoken."
On Sunday he said: "We should ask ourselves if we're doing all we can to stem the tide of gun violence that claims too many lives across this country on a daily basis. We should ask ourselves, as individuals and as a society, how we can prevent future tragedies like this. As citizens, that's a job for all of us. That's a way to honor Trayvon Martin."
Los Angeles Times