Several thousand people marched from Union Square to Times Square in New York after George Zimmerman, a local Neighbourhood Watch captain in Florida, was found not guilty for the death of black teenager Trayvon Martin.
Protests broke out in several cities in the United States on Sunday evening as people voiced their disappointment and frustration at the verdict.
Marches of varying sizes also erupted in San Francisco, Chicago, Washington, Atlanta, Oakland and Philadelphia following the reading out of the verdict late on Saturday night.
Al Jazeera's Matthew Cassel, reporting from New York said the demonstration was 10,000 strong by the time it reached Times Square.
Protesters were chanting slogans such as "Trayvon's dead, Zimmerman's free, that's what they call democracy."
A jury in the town of Sanford, where the shooting occurred, found Zimmerman not guilty of shooting dead Martin, a 17-year-old unarmed teen on the night of February 26 last year.
Martin's parents were not in the court during the reading of the verdict. His father, Tracy Martin, however, later tweeted that his son would have been proud of the fight put up for him.
"Even though I am broken hearted my faith is unshattered," he wrote in one tweet. "Together can make sure that this doesn't happen again," he said, in another.
Elsewhere in the US, demonstrations remained largely peaceful, despite the strength of feeling felt by those marching.
Al Jazeera's Yasmine Ryan, reporting from Oakland, California, said the demonstrators were feeling let down by the legal system.
Paola Baccette, a professor at UC Berkelely, told our correspondent that she felt "pure outrage" over yesterday's ruling.
"We live in a racist country and most of the murderers of young black men walk free."
US President Barack Obama said on Sunday that Martin's death was a tragedy for the country and called for calm after Zimmerman's acquittal.
"We are a nation of laws, and a jury has spoken. I now ask every American to respect the call for calm reflection from two parents who lost their young son," said Obama.
While Zimmerman has been acquitted of criminal charges, the US justice department confirmed on Sunday that it was investigating the possibility of filing civil rights charges against the Florida resident.
'No justice, no peace!'
The criminal trial had riveted the United States for weeks, and emotions came to a boiling point as news of the verdict spread.
Prominent US rights activists such as Reverend Jesse Jackson appealed for calm.
"Avoid violence, it will lead to more tragedies. Find a way for self-construction not deconstruction in this time of despair," he wrote on Twitter.
Even though I am broken hearted my faith is unshattered [...] Together can make sure that this doesn't happen again.
Martin's parents have long called for non-violent demonstrations, quoting civil rights icon Martin Luther King and the Bible.
Several hundred demonstrators marched peacefully amid a heavy police presence in downtown San Francisco soon after the verdict was announced. Many carried signs with slogans such as "The people say guilty" and "The whole system is racist".
In New York City, crowds carrying homemade signs and blowing whistles marched and gathered in the iconic Times Square.
In Chicago, to the cry of "No justice, no peace! No racist police!" a crowd of activists held a noisy downtown rally, the Chicago Tribune reported.
Los Angeles police declared a "citywide tactical alert" when about 200 demonstrators gathered at a park in a historical black neighbourhood to demonstrate, but police later told local media that it was as a precaution, and that there had been no acts of violence.
Dozens of mostly African-American youths marched chanting slogans in Washington DC, the US capital. They were followed closely by patrol vehicles.
A crowd of several hundred gathered all day on Saturday outside the courthouse in Sanford - and many were outraged when the verdict was read.
"It's the end of our justice system," said Ashton Summer, a 20 year-old Puerto Rican. "Justice is not equal for everyone."
Several further demonstrations have been planned in major cities across the United States for Sunday.
"We are very saddened by the jury's verdict," said Martin family attorney Benjamin Crump after the verdict was read. "The family is heartbroken."
Rights activist Al Sharpton posted a statement on Facebook describing Zimmerman's acquittal as "a slap in the face to the American people".
"We intend to ask the Department of Justice to move forward as they did in the Rodney King case and we will closely monitor the civil case against Mr Zimmerman," said Sharpton.
Rodney King was an African-American man who was beaten by Los Angeles police following a car chase in April 1991. The beating was videotaped and aired on television, sparking widespread outrage.
Throughout the trial, Zimmerman said his actions had been in self-defence.
In Florida, the "Stand your ground" law means people can justify shooting people when they feel threatened, rather than retreating if the option is available.
Source: Al Jazeera and agencies