The demonstrations began around noon at federal buildings across the country. They came a week after the volunteer, George Zimmerman, was acquitted by a court in Florida in Martin's killing; days after angry protests erupted in the wake of that verdict; and hours after President Barack Obama said, in a heartfelt address, that "Trayvon Martin could have been me 35 years ago."
The events were largely peaceful, though attendees in many cities faced sweltering heat - with temperatures in some places reaching into the 80s and 90s. In some places, like Dallas, there were news reports that up to 25 demonstrators were treated by medical personnel for heat-related problems. In Atlanta, storms also bedeviled protesters, leading to the cancellation of an evening rally in a nearby suburb.
Martin's father, Tracy, addressing dozens of people outside the federal courthouse in Miami, said, "I vowed to Trayvon when he was laying in his casket that I would use every ounce of energy in my body to seek justice for him."
"I will continue to fight for Trayvon until the day I die," he added. "Not only will I be fighting for Trayvon, I will be fighting for your child as well."
Isabel Eugene, 16, who also attended the rally in Miami, said: "Before Trayvon Martin, we took precautions, but now it's worse. It could have been my brother."
At a rally in New York, many attendees held umbrellas to shield themselves from the overpowering heat. As the crowd of thousands shouted, "We're all Trayvon Martin," the Rev. Al Sharpton, one of the organizers of the gatherings, spoke, saying Martin's death should prompt a larger movement.
Sharpton then announced a plan to hold a protest in the capital in August to commemorate the 50th anniversary of the March on Washington for Jobs and Freedom, where the Rev. Dr. Martin Luther King Jr. delivered his famous "I Have a Dream" speech.
Sharpton said he wanted to ensure an aggressive federal investigation of Zimmerman and fight against Florida's broad self-defense laws. "Last Saturday we cried," he said, "but this Saturday we march."
"We're going to keep the focus on the Justice Department because Trayvon Martin had the civil right to go home that day," he added. "We cannot have a society where any one of our children can be taken based on someone feeling they had the right to stand their ground. Well, what about Trayvon's right to stand his ground? And what about our right to stand our ground?"
He also reiterated a point that Obama had made in his speech the previous day. "You don't know the humiliation of walking in a department store and you're assumed to be a suspect rather than a customer," Sharpton said. "You don't know the humiliation of being guilty till proven innocent. You don't know the humiliation of how people judge you based on what your skin color is."
Martin's mother, Sybrina Fulton, who also attended the New York rally, stepped up to speak next, choking on her words as she faced the swelling crowd.
"Trayvon was a child, and I think sometimes it gets lost in the shuffle because as I sat in the courtroom, it made me think that they were talking about another man," Fulton said. "And it wasn't, it was a child."
She later added: "Of course we're hurting. Of course we're shocked and disappointed, but that just means that we have to roll up our sleeves and continue to fight."
In Atlanta, the site of repeated protests since the verdict, several thousand demonstrators withstood torrential rains at a rally that focused on demanding action from the Justice Department in the wake of Obama's comments Friday.
"I need new federal charges along with that feel-good speech," said Marcus Coleman, 39, told a crowd that roared its approval.
The protest, on the steps of the city's federal building, drew two of King's children, who invoked their father's legacy and words while urging the demonstrators to press forward ahead of the 50th anniversary of the March on Washington next month.
In an interview before his appearance, Martin Luther King III said he believed the verdict had led to an unusually introspective and widespread national conversation about race and the criminal justice system.
"This is a tone-setting time that at least sets up a framework for dialogue that has not existed in a long time," King said. "People are not as frustrated about the verdict. It's about consistent, systemic kinds of things that don't get reported and happen every day in courtrooms across America."
Charlotte Wilson, a 70-year-old retiree from Atlanta, was among those who stood in the front row of the protest and said she believed the next challenge for the demonstrators would be maintaining their intensity.
"This is powerful, but we have to continue," Wilson said. "We have to stick together."
But the Rev. Markel Hutchins, one of the event's organizers, said he was not anxious that the momentum would fade.
"There's a new level of energy," Hutchins said. "There's a new level of enthusiasm that I personally have not seen since the days of the civil rights movement. Perhaps Trayvon Martin's death - and perhaps even the not guilty verdict in the George Zimmerman trial - has inspired and ignited a movement of people who, frankly, needed to be moved."
Several hundred marched also through downtown Los Angeles. Some carried placards that read: "Open season on the black man" and "Federal charges for Zimmerman."
Cindy Holdorff, 46, who is white, attended the march with her 9-year-old adopted son Sam, who is black. Holdorff said she had brought Sam in order to prepare him for the realities of racial prejudice.
"He's been learning about slavery in the classroom, so I said to him, 'Slavery is over, but unfortunately people still judge people by their color,'" she said, adding that that is what she believe happened to Trayvon Martin. "We're going to go fight because we don't want this to happen again."
At similar rallies in Washington and other cities large and small, crowds of hundreds held up signs reading "I am not a suspect" and "Trayvon Martin has civil rights."
"Before Trayvon Martin, we took precautions, but now it's worse," Isabel Eugene, 16, said at the rally in Miami. "It could have been my brother."
On July 13, after three weeks of testimony, a six-woman jury rejected the prosecution argument that Zimmerman had deliberately pursued Martin because he presumed the hoodie-clad teenager was a criminal and instigated the fight that led to the killing.
Zimmerman said he shot Martin on Feb. 26, 2012, in self-defense after the teenager knocked him to the ground, punched him and slammed his head repeatedly against the sidewalk. In finding him not guilty of murder or manslaughter, the jury agreed that Zimmerman could have been justified in shooting Martin because he feared great bodily harm or death.
The Justice Department restarted a federal investigation into the case after the acquittal to determine whether the evidence "reveals a prosecutable violation of any of the limited federal criminal civil rights statutes," it said in a statement.
To win a conviction, the government would have to prove that Zimmerman acted willfully to violate Martin's civil rights, people familiar with such cases have said.
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