WASHINGTON - Family members of young, unarmed black men killed by police - from Michael Brown and Eric Garner in recent months, to Amadou Diallo more than 15 years ago - packed a stage in front of the U.S. Capitol on Saturday, using a shared forum to urge thousands of supportive marchers to keep up the pressure for changes to the criminal justice system.

The march in Washington coincided with demonstrations across the country, from Fifth Avenue in New York to the streets of San Francisco and the steps of the Massachusetts Statehouse - mostly peaceful protests although about two dozen people were arrested in Boston for disorderly conduct.

"My husband was a quiet man, but he's making a lot of noise right now," Esaw Garner, widow of Eric Garner, said in Washington. Her husband, 43, died in July after being put in a chokehold by New York City police during an arrest for allegedly selling loose, untaxed cigarettes. "His voice will be heard. I have five children in this world and we are fighting not just for him but for everybody's future, for everybody's past, for everybody's present, and we need to make it strong."

Nationally, marchers chanted "I can't breathe!" and "Hands up, don't shoot!" and waved signs reading "Black lives matter!" Demonstrators also staged "die-ins" as they lay down across intersections and in one city briefly scuffled with police blocking an onramp to a highway.

Organizers had predicted 5,000 people at the Washington march, but the crowd appeared to far outnumber that. They later said they believed as many as 25,000 had shown up. Washington police do not release crowd estimates.

Garner's mother, Gwen Carr, called the demonstrations a "history-making moment."

"It's just so overwhelming to see all who have come to stand with us today," she said. "I mean, look at the masses. Black, white, all races, all religions. ... We need to stand like this at all times."

There also were speakers from the family of Tamir Rice, a 12-year-old killed in Ohio as he played with a pellet gun in a park.

Kadiatou Diallo, whose son Amadou was shot and killed in the Bronx in 1999 by four police officers, reflected on how the same issues being debated today were debated when her son was killed. "We've been there so many times," she said. "Today we are standing still and demanding the same thing."

The Rev. Al Sharpton helped organize the marches. "Members of Congress, beware, we're serious ...," Sharpton said in Washington. "When you get a ring-ding on Christmas, it might not be Santa; it may be Rev. Al coming to your house."

Protests - some violent - have occurred around the nation since grand juries last month declined to indict the officers involved in the deaths of Garner and Michael Brown, 18, shot and killed by a white police officer in Ferguson, Mo., a suburb of St. Louis.

Washington and U.S. Park Police said they had made no arrests in the capital protests, though a small group of protesters split off after the march and briefly occupied various intersections in downtown Washington.

The noisy march through the heart of Manhattan swelled to at least 25,000 people, police said. It snarled traffic but remained peaceful, with no arrests reported by late afternoon. On Saturday night, some protesters marched across the Brooklyn Bridge, blocking traffic in both directions.

Hundreds of protesters took to the downtown streets of San Francisco on Saturday, while at the University of California, Berkeley, police removed life-sized photographs of lynching victims that had been hung at the campus. Investigators believe they were connected to a smaller protest in Berkeley at noon.