The vigils, observed in more than 90 cities as part of a National Moment of Silence, came days after the shooting death of 18-year-old Michael Brown and the death of a New York man caused by a police officer's chokehold.
In downtown St. Louis, in a tiny park near the Gateway Arch, several hundred people, seemingly an equal number whites and blacks, gathered in Brown's memory.
The site is a short drive from suburban Ferguson, where Brown was killed, stoking racial unrest. In Ferguson, two-thirds of the 21,000 residents are black and all but three of the 53 police officers are white.
The St. Louis gathering was peaceful in contrast with a night of looting and clashes between demonstrators and police in Ferguson earlier in the week.
The attendees included Brown's mother, Lesley McSpadden, who didn't address the crowd but waved, drawing applause as she wiped away tears.
The observance was among many staged nationwide, each with a minute of silence for Brown and others who died at the hands of police.
Bishop Elliott Coleman, of St. Louis' El Bethel Temple Church, said in opening the observance there that people were coming together "for this reason of healing."
"Realize there are tears in every city, tears in homes, tears in the eyes of young people, tears in the eyes of old people," Coleman said. "The tears need to be wiped away, and the hearts need to be healed."
In New York, thousands of people peacefully gathered in Manhattan's Times Square and Union Square, invoking the rallying cries "hands up, don't shoot" and "I can't breathe," alluding to the death of 43-year-old Eric Garner, who was arrested on Staten Island on suspicion of selling loose, untaxed cigarettes and was placed in an officer's chokehold.
Garner, who had asthma, can be heard on tape shouting "I can't breathe!" and died a short time later.
The police commissioner in New York has said officers will be retrained on the use of force. Police in Ferguson have said Brown was shot after an officer encountered him and another man on the street and one of the men pushed the officer into his squad car and physically assaulted him.
Antonia Moe, who attended the Union Square vigil with her 12-year-old son, said incidents like Brown's death have changed the way she talks to her son about being black.
"When things like this happen you kind of have to remember to remind him that some of the rules that apply to others don't necessary apply to you," she said.
In Orlando, Florida, about 15 miles outside the Sanford suburb where unarmed black teenager Trayvon Martin was shot and killed by neighborhood watch leader George Zimmerman in 2012, a multicultural crowd of about 100 people gathered in front of a park amphitheater.
One woman carried a sign that read: "No Justice, No Peace. We Stand With Ferguson." Another man's sign said: "Hands up. Don't Shoot! RIP Mike Brown."
In Chicago, hundreds of people gathered downtown at Daley Plaza, where chants of "No justice! No peace! No racist police!" were heard.
In Nevada, about 40 people gathered outside the federal courthouse in Reno, and dozens gathered in Seattle, holding up signs that read "Unite Against Racism" and "Solidarity With Ferguson."
Associated Press writer Jim Suhr in St. Louis contributed to this report.