The Rev. Al Sharpton's National Action Network organized the "Justice for Trayvon" rallies and vigils outside federal buildings in at least 101 cities one week after a jury delivered the verdict for George Zimmerman in Martin's 2012 death in a gated central Florida community.
"No justice! No peace!" participants chanted. Some sang hymns, prayed and held hands. Many held signs - in Los Angeles, one read, "This is Amerikkka: From Dred Scott to Emmett Till to Trayvon Martin, black people have no rights that white people are bound to respect."
The case has become a flashpoint in separate but converging national debates over self-defense, guns, and race relations. Zimmerman, who successfully claimed that he was protecting himself when he shot Martin, identifies himself as Hispanic. Martin was black.
In Atlanta, speakers noted that the rally took place in the shadows of federal buildings named for two figures who had vastly differing views on civil rights and racial equality: Richard B. Russell was a Georgia governor and U.S. senator elected in the Jim Crow South; the Rev. Martin Luther King Jr. is the face of African-Americans' civil rights movement.
"What's so frightening about a black man in a hood?" said the Rev. Raphael Warnock, who now occupies the pulpit at King's Ebenezer Baptist Church.
In New York, hundreds of people - including Martin's mother, Sybrina Fulton, and music superstars Jay-Z and Beyonce - gathered in the heat.
Fulton told the crowd she was determined to fight for changes needed to ensure that black youths are no longer viewed with suspicion because of their skin color.
"I promise you I'm going to work for your children as well," she told the crowd.
Earlier Saturday, at Sharpton's headquarters in Harlem, she implored people to understand that the tragedy involved more than Martin alone. "Today it was my son. Tomorrow it might be yours," she said.
In addition to pushing the Justice Department to investigate civil rights charges against Zimmerman, Sharpton told supporters In New York that he wants to see a rollback of stand-your-ground self-defense laws.
"We are trying to change laws so that this never, ever happens again," Sharpton said. His daughters, Ashley and Dominique Sharpton, were scheduled to lead a follow-up march Sunday in Harlem.
Stand-your-ground laws are on the books in more than 20 states, and they go beyond many older, traditional self-defense statutes. In general, stand-your-ground laws eliminate a person's duty to retreat, if possible, in the face of a serious physical threat.
Zimmerman didn't invoke stand-your-ground, relying instead on a traditional self-defense argument, but the judge included a provision of the law in the jurors' instructions, allowing them to consider it as a legitimate defense.
Neither was race discussed in front of the jury. But the two topics have dominated public discourse about the case, and came up throughout Saturday's rallies.
"It's personal," said Cincinnati resident Chris Donegan, whose 11-year-old son wore a hoodie to the rally, as Martin did the night he died. "Anybody who is black with kids, Trayvon Martin became our son."
In Indianapolis, the Rev. Jeffrey Johnson told roughly 200 attendees that the rallies were about making life safer for young black men who are still endangered by racial profiling.
Johnson compared Zimmerman's acquittal to that of four white officers in the beating of black motorist Rodney King in 1992.
"The verdict freed George Zimmerman, but it condemned America more," said Johnson, pastor of the Eastern Star Church in Indianapolis and a member of the board of directors of the National Action Network.
In Miami, Tracy Martin spoke about his son.
"This could be any one of our children," he said. "Our mission now is to make sure that this doesn't happen to your child."
He recalled a promise he made to his son as he lay in his casket. "I will continue to fight for Trayvon until the day I die," he said.
Shantescia Hill held a sign in Miami that read: "Every person deserves a safe walk home." The 31-year-old mother, who is black, said, "I'm here because our children can't even walk on the streets without fearing for their lives."
Attorney General Eric Holder announced this week that his department would investigate whether Zimmerman could be charged under federal civil rights laws. Such a case would require evidence that Zimmerman harbored racial animosity against Martin.
Most legal experts say that would be a difficult charge to prove. Zimmerman's lawyers have said their client wasn't driven by race, but by a desire to protect his neighborhood.
Associated Press writers Phillip Lucas in Atlanta, Charles Wilson in Indianapolis, Amanda Lee Myers in Cincinnati, Christine Armario in Miami and Verena Dobnik in New York contributed to this report.