Advocates want federal civil rights charges against Zimmerman, who was acquitted Saturday in Martin's 2012 shooting death.
The Rev. Al Sharpton said Monday that his organization will hold vigils and rallies in 100 cities Saturday in front of federal buildings.
The Justice Department has said it's considering whether federal prosecutors should file criminal civil rights charges now that Zimmerman has been acquitted in the state case. The department opened an investigation into Martin's death last year but stepped aside to allow the state prosecution to proceed.
Sunday's demonstrations, held in cities from Florida to Wisconsin, attracted anywhere from a few dozen people to a more than a thousand.
At a march and rally in downtown Chicago attended by about 200 people, 73-year-old Maya Miller said the case reminded her of the 1955 slaying of Emmett Till, a 14-year-old from Chicago who was murdered by a group of white men while visiting Mississippi. Till's killing galvanized the civil rights movement.
"Fifty-eight years and nothing's changed," Miller said, pausing to join a chant for "Justice for Trayvon, not one more."
In New York City, more than 1,000 people marched into Times Square on Sunday night, zigzagging through Manhattan's streets to avoid police lines. Sign-carrying marchers thronged the busy intersection, chanting "Justice for! Trayvon Martin!" as they made their way from downtown Union Square, blocking traffic for more than an hour.
In San Francisco and in Los Angeles, where police dispersed an earlier protest with beanbag rounds, police closed streets Sunday.
President Barack Obama, Sharpton and the Rev. Jesse Jackson have urged calm. In Oakland, Calif., during protests that began late Saturday night, some angry demonstrators broke windows, burned U.S. flags, vandalized a police squad car and spray-painted anti-police graffiti.