Family members of some of those who have been killed were in attendance.

“My son was 12, just a baby,” said Samaria Rice, the mother of Tamir Rice, who was shot to death in Cleveland after the police saw him with what turned out to be a toy gun that fired plastic pellets. “He is here with me right now, and this is what he would want me to do,” she added. “He had a promising future. All the families, we share the same pain.”

“I’m marching for everyone’s sons and daughters,” said Esaw Garner, the widow of Mr. Garner.

The march follows protests around the country over recent grand jury decisions not to indict the officer who shot Mr. Brown in Ferguson, Mo., or the one who placed Mr. Garner in a fatal hold on Staten Island. Scores of protesters on Saturday came from Ferguson.

On Friday, the death of Tamir Rice was formally ruled a homicide, according to a county autopsy report; he died in November when an officer shot him within seconds of arriving on the scene.

Protesters in recent weeks have occupied parks, city streets, train stations and retail stores to draw attention to the use of force by the police. In New York, thousands of people have tried to close major roads, bridges and tunnels, disrupting the rhythms of the city. In Berkeley, Calif., an Amtrak train was forced to stop, a central freeway was closed for hours and regional commuter trains were halted.

“For me to come out from my comfortable cocoon in this stage of my existence is something,” said Cynthia Pace, a consultant from Baltimore who was marching here on Saturday. “We have enough money, our children are highly educated, all of these things. But I can’t imagine how we in the American public can see something on TV and 12 people see it differently. There is something wrong with this picture.”