Superstar couple Jay Z and Beyonce joined protesters across the United States on Saturday as anger continued to simmer over a not-guilty verdict in the shooting death of black teenager Trayvon Martin.

The call for nationwide vigils designed to press for federal action in the emotionally charged case drew thousands to one New York rally.

Elsewhere turnouts were modest with the crowds drawn overwhelmingly from the black community, including in Miami where the slain 17-year-old's father spoke.

"The death of my son, we believe, has to make changes in our society and repeal the laws that allow to kill somebody just because someone thinks (he) is suspect," Tracy Martin said.

The surprise appearance of hip hop star Jay Z and his wife, pop superstar Beyonce Knowles, helped swell the crowd at a Manhattan protest addressed by Trayvon's mother, Sybrina Fulton, and civil rights leader Al Sharpton.

Briefly overcome with emotion as she thanked the crowd for their support, Fulton vowed to ensure her son's death would not be in vain but pleaded for protests to be kept peaceful.

"Trayvon would be proud," she told the crowd. "Not only do I have to do what I have to do for Trayvon, I'm going to work for your children as well."

In Miami, between 300 and 500 people turned out for the rally, but a heavy downpour quickly dispersed the crowd just after noon -- and some participants expressed disappointment at the small, mainly black turnout.

"You see only black people, when this should be a Hispanic, white issue," said Marie Falaise, 39, a Miami resident born in Haiti.

In Chicago, speaker after speaker compared Martin's death to that of city native Emmett Till, whose racially-motivated murder in Mississippi was a pivotal moment in the civil rights movement.

Grandmother Dorothy Clay, 66, wore a pack of Skittles on her hat in honor of Martin, who was out buying the sweets on the night he died.

She said she fears for her kids and five grandchildren in a country where a "cowardly, wannabe hero-vigilante" can get away with killing an unarmed teenager.

In a message posted on her website, Beyonce also recalled 14-year-old Till's brutal killing in 1955. "We must fight for Trayvon the same way the generation before us fought for Emmett Till," she wrote.

The protests came a day after President Barack Obama publicly identified with Martin and the deep frustrations felt among African-Americans over the verdict. "Trayvon Martin could have been me 35 years ago," Obama told reporters Friday.

The president's remarks -- his most expansive since a Florida jury's decision one week ago to acquit neighborhood watch volunteer George Zimmerman -- were applauded by many at Saturday's vigils.

"I felt that in the past he has been hesitant and tried to ride the line and not offend anyone," Sonya Olange, a 45-year-old mother of two at the Manhattan rally, told AFP.

"But his job as president is to set the tone for changing these laws and, as a black man, he did what he should have done."

The 29-year-old Zimmerman, who said he acted in self defense, fatally shot Martin in the chest on the rainy night of February 26, 2012, during an altercation in a gated community in Sanford, Florida.

A jury of six women, all but one white, cleared him of second-degree murder and manslaughter charges.

"George Zimmerman started the fight and George Zimmerman ended the fight," Martin's mother said in New York. "My son died without even knowing who his killer was. Today it was my son. Tomorrow it might be yours."

The vigils outside court houses and government buildings were organized to maintain public pressure for a federal civil rights investigation into the killing.

Organizers are targeting "Stand Your Ground" laws like Florida's, which assert that citizens can use lethal force -- rather than retreat -- if they sense their lives are at risk.

"We are not going to be silent, we'll stand up for what's right, we'll stand up for justice," Sharpton said at the New York rally. "If you mess with one of the children, the family has to come in and protect them."

"Like Jay Z told me, he is a father. Beyonce is a mother. We all have children, and we feel the fear and apprehension. The laws must protect everyone," Sharpton added.

Obama did not comment directly on the Florida verdict but he called for a review of the controversial state self-defense laws.

"I just ask people to consider if Trayvon Martin was of age and armed, could he have stood his ground on that sidewalk?" Obama said.

"And do we actually think that he would have been justified in shooting Mr Zimmerman who had followed him in a car because he felt threatened?"

The teen's parents said they were "deeply honored and moved" by Obama identifying himself with Trayvon. "This is a beautiful tribute to our boy," they said in a statement.