(CNN) -- The families of Trayvon Martin, Michael Brown, Tamir Rice, Eric Garner and other unarmed black men felled by bullets or police force filled a podium in the nation's capital.
"This is a history-making moment," Gwen Carr told a crowd of thousands. Her son, Eric Garner, died after a police officer put him in a chokehold.
"It's just so overwhelming to see all who have come to stand with us. Look at the masses -- black, white, all races, all religions. ... We need to stand like this at all times."
Invoking the familiar names of black men who died at the hands of police, tens of thousands marched throughout the nation on Saturday to protest what they see as rampant racial injustice.
The throngs -- young and old, black and white -- took to the streets in major cities, including New York. In Washington, a crowd of thousands snaked up Pennsylvania Avenue to the Capitol in a protest that evoked memories of past civil rights marches.
The crowds appear to represent a burgeoning movement sparked by the decisions of grand juries in Missouri and New York not to indict white police officers in the deaths of two unarmed black men.
'Sea of people'
Lesley McSpadden, the mother of Michael Brown, a teenager killed in Ferguson, Missouri, marveled at the crowds.
"What a sea of people," she said. "If they don't see this and make a change, then I don't know what we got to do."
Kadiatou Diallo, whose son Amadou was shot 41 times by four New York police officers in 1999, held up a Time magazine cover about her son's slaying. The officers were prosecuted on second-degree murder and other charges but were acquitted by a jury.
"Today, 15 year later, we are standing together and demanding the same thing," she said of police brutality. "Think about that for a moment. Think about all the young men taken from us."
Looking out at the throng before her, she added, "This is not the end, because of the sea of people who stood up."
Similar demonstrations were held Saturday in New York, Boston and San Francisco.
In a march that started at the Massachusetts State Capitol, thousands snarled traffic on the streets of Boston. Near Nashua Street, police arrested 23 people on disorderly conduct charges, according to the Massachusetts State Police. One man also was charged with assault and battery on a police officer.
From New York's Washington Square Park, thousands of demonstrators marched through traffic-choked streets to police headquarters. They carried brightly colored signs and chanted "I can't breathe" and "How do you spell racist -- NYPD."
The march was largely peaceful, although a protester from a splinter group hurled a garbage can through the window of an NYPD traffic car.
'Part of History'
"I stand here as a black man who is afraid of the police," said marcher Ahmad Greene-Hayes, "who is afraid of never knowing when my life might end, never knowing when I might be ... gunned down by a vigilante or a security guard or a police officer. ... That fear, that trepidation is rooted more so in my connection to my ancestors ... who were enslaved, those who were beaten during the civil rights movement. ... So there's a longstanding history that I'm connected to."
Michaela Angela Davis, a writer and frequent CNN guest who participated in the march, said the demonstrations signal a lasting movement.
"They're tired," she said of protesters. "There's nothing left. They have nothing left to lose. I feel like what's so powerful about being here today and why I wanted to be here today -- the collective self-esteem that's happening with these young people is really powerful. They know they are part of history."
In Washington, many carried signs with now familiar messages: "#Black lives matter," "Hold cops accountable" and "I can't breathe." The latter is what Garner is heard saying repeatedly to police in a video as they appear to hold him down.
"I'm here for the voiceless, for those that have died for the injustice and systematic racism that is happening on this land," said Shanna Lawrie, her hands in the air as she marched to the Capitol. "It's systematic racism that is instilled in our government."
Busloads of young people left New York's Harlem neighborhood before dawn to participate in the Washington march. Others came from Ferguson, Missouri.
"We come in peace but we come strong," said Ashley Sharpton, daughter of the Rev. Al Sharpton, an organizer of the Washington march. "We come with demands. We want the government to get involved."
Protesters are demanding a more aggressive federal response to a spate of racially charged police shooting incidents. They say their fight is a human rights struggle.
Washington resident Anthony Passmore said he was marching for the future of his child and others.
"I want a future for them to actually be able to do what they want to do, be what they want to be," he said. "They say this is the land of opportunity, the land of freedom. Let them live right and not be judged."
'Week of outrage'
Another marcher was Samaria Rice, whose 12-year-old son was shot to death by a Cleveland police officer while carrying a toy handgun in a park. On Friday, the Cuyahoga County Medical Examiner's Office said his death was ruled a homicide.
"He is here with me right now, and this is what he would want me to do," she told the crowd. "He had a promising future. All the families, we share the same pain."
Marc Morial, head of the National Urban League, enumerated a list of protester demands that included body and dashboard cameras for all police officers, special prosecutors to investigate police misconduct and laws against racial profiling.
New York City Councilman Jumaane Williams, who marched in Washington, said the protest needed to move beyond police brutality and focus on issues such as housing, unemployment and better schools.
"That's where we need to steer the conversation," he said. "That's where the anger is."
In recent weeks, thousands have marched and stopped traffic in protests surrounding the controversial deaths of Brown near St. Louis and Garner in New York.
Saturday's demonstrations cap what organizers are calling a nationwide "week of outrage."
"Our message is very simple," said Carl Dix, founder of the Stop Mass Incarceration Network, one of the main organizers of the protests. "Since the 'normal routine' of America has always included murder of black and Latino people by law enforcement, this week, that 'normal routine' must be disrupted."
Eric Garner Jr., the son of the man who died after a New York police officer held him in what appeared to be a chokehold, said he was proud of the protests.
"It's amazing how everybody (is) doing this. My father and I appreciate it," Garner told CNN's "Erin Burnett OutFront."
Some peaceful protests across the country this week have been marred by bouts of violence and crowds that disrupted thousands of motorists by shutting down freeways.
Daniel Pantaleo, the police officer shown on video wrestling Garner to the pavement with his arm around his neck, spoke with internal affairs investigators this week. A New York grand jury decided last week not to indict him in Garner's death.
"He indicated he never used a chokehold," said Stuart London, his attorney. "He used a takedown technique he was taught in the academy. He said he never exerted any pressure on the windpipe and never intended to injure Mr. Garner."
In Missouri, Officer Darren Wilson -- who testified to a St. Louis County grand jury that he shot Brown in August after the 18-year-old tried to take his gun and then charged at him -- resigned from the Ferguson Police Department last month.