In a strong display of unity since the shooting of an unarmed black teenager in Ferguson, Mo., the relatives of African Americans killed by officers in incidents dating back more than a decade shared a stage Saturday to call on Congress to make changes in the criminal justice system.

Speaking in the cold afternoon air, mothers and fathers of those who were killed said their children were let down by prosecutors and grand juries that did not indict police officers in many of the cases, saying the attorneys have too close a relationship to police to fairly represent shooting victims.

On the stage, passing a microphone like a baton, one speaker after another addressed tens of thousands of racially diverse marchers at the “Justice for All” rally organized by the Rev. Al Sharpton’s National Action Network. They vowed to continue protesting until Congress takes action, such as requiring a special prosecutor to investigate controversial police shootings.

“We will come here as many times as it takes,” said Gwen Carr, the mother of Eric Garner, who died in Staten Island in July after an officer held him in a chokehold as the 43-year-old said, “I can’t breathe.” The arrest was caught on video, and a grand jury’s decision earlier this month not to indict the officer shocked liberals and conservatives alike.

The Washington march was one of many that took place across the nation Saturday.

Bishop D. Demond Robinson and 25 residents of Ferguson took an 800-mile road trip from Missouri to Washington to march with other protesters from around the country in opposition of police brutality. (Whitney Leaming/The Washington Post)

Under the slogan “National Day of Resistance,” rallies were held in New York, Boston, Los Angeles, San Francisco, Bloomington, Ind., and Lexington, Ky., among other places, part of a groundswell of protests since August.

A “Millions March” in New York started big and grew much bigger within hours, from a densely packed crowd in two city blocks to a mass as far as the eye could see as the march closed on its destination near New York University.

The crowd in New York reflected the diversity of the city. “I’ve never seen something so beautiful,” said Josh Toney, an African American man in his 20s. “Seeing the Asian community, seeing union workers, seeing people who probably don’t even speak English.”

Like the D.C. march, police declared the New York march peaceful with no arrests. In Boston, however, police made multiple arrests when protesters clashed with officers.

The demonstrations sweeping the country have their roots in the months-long protests in Ferguson, where what happened between Michael Brown, 18, and the officer who shot him remains contested by many. A grand jury’s decision in November not to indict the officer sparked more unrest in the St. Louis suburb where protesters and police clashed in the days immediately after the shooting.

The grand jury decision in Garner’s death was announced a little more than a week later, and has further intensified the nation’s focus on race and policing. The video left many expecting an indictment.

His mother called the support from other families and from the thousands of demonstrators around the country overwhelming. “I mean, look at the masses. Black, white, all races, all religions. We need to stand like this at all times,” Carr said moments after she was engulfed by a hug from Sybrina Fulton, the mother of Trayvon Martin. Her son was shot in the chest in 2012 during a struggle with a self-appointed neighborhood watch volunteer.

A crowd of thousands jammed between Third and Fifth streets NW on Pennsylvania Avenue listened as each speaker thanked them for turning out.

“We will get justice for our children, believe that,” said Samaria Rice, the mother of 12-year-old Tamir Rice, who was shot to death by a Cleveland police officer while playing with a toy gun. A video of the shooting showed that the officer shot the child within seconds of arriving at the scene, no questions asked.

“I want to thank the nation for supporting us,” Rice said, as her hand trembled and her voice cracked. “That’s the only way I’m standing today, that I’m standing right now.”

The father of another victim, John Crawford III, who was shot dead in a Wal-Mart store by police officers in a Dayton, Ohio, suburb, said the same criminal justice system he works for “is the same system I’m receiving injustice from.”

Police said they thought a BB gun the younger Crawford carried was lethal. A video of the shooting shows that he was on a cellphone before he was shot. A grand jury decided not to indict the officers who fired the shots.

“My son was murdered in the biggest retail store in the world,” Crawford said. “These cases should be open and shut. Let’s stay focused on that. Don’t forget my son’s name. They will all be vindicated.”

Lesley McSpadden, the mother of Michael Brown, said “it speaks volumes” that the march was so large and diverse. She said those who have died have been immortalized. “It is almost like you don’t need to say any words, you can just look out and see all of the people. They shot our sons, but they did not die.”

Several protesters walked up to Michael Brown Sr. seeking hugs, handshakes or a picture. Brown tried to be accommodating to the many requests, at one point showing a young man how to take a selfie.

“This means the world to me, to see everyone coming together for a common cause,” Brown said.

When it was her turn to speak, Sybrina Fulton said shootings of unarmed black men and children are nothing new.

“This is something that’s been going on a long time. It’s just that some people have just woke up.”

Fulton’s point was punctuated by the remarks of Kadiatou Diallo, the mother of Amadou Diallo, who was shot 19 times in 1999 by New York plainclothes police officers. They said they thought the wallet he pulled out to show his identification was a gun.

Diallo held up a dated magazine cover featuring her son with the caption, “Cops, Brutality and Race.” “Today — 16 years later — we are standing here saying the same thing,” she said.

Like two other speakers, Fulton took a moment to thank someone else. “This guy to my left, the Rev. Sharpton. Where would we be without him? People don’t understand. They talk about we’re not together. I look around. We are together.”

But on that issue there was wide disagreement. Few if any young protest organizers who fueled the movement after Michael Brown was shot were invited to speak. Until Saturday, protests had been organic and original, and not surprisingly the burgeoning movement has shown generational fractures from its earliest days in Ferguson.

At one point, before the procession left its gathering place at Freedom Plaza near D.C. city hall, a group of young demonstrators mostly from Ferguson seized the stage. Opposed to Sharpton, who they view as a celebrity activist seeking to take over a movement they started, they said young advocates who did the heavy lifting should be at the forefront of the march.

But when Sharpton took the stage at the Capitol, he urged the marchers not to let “provocateurs” divide them, by generation or race. “This is not a black march or a white march,” Sharpton said. “This is an American march so the rights of all Americans are protected. I’m inspired when I see white kids holding up signs saying ‘Black Lives Matter.’ ”

What’s next? Sharpton and others said they would press Congress to pass legislation requiring body and dashboard cameras for all police officers, special prosecutors to investigate police misconduct and beefed-up laws against racial profiling. Sharpton is also pushing for a new division within the Justice Department to deal with the tensions between African Americans and police departments.

“Members of Congress, beware,” Sharpton said. “We’re serious. When you get a ring-ding on Christmas, it might not be Santa. It may be Rev. Al coming to your house.”

Correction: An earlier version of this story misspelled the first name of Trayvon Martin’s mother, Sybrina Fulton.

Keith Alexander, Moriah Balingit, Hamil Harris, Mariam Baksh and Whitney Leaming in Washington and Philip Bump in New York contributed to this report.