The Florida case has become a flashpoint in separate but converging national debates over self-defense laws, guns, and race relations. Zimmerman, who successfully claimed self-defense, identifies as Hispanic. Martin was black.
The Rev. Al Sharpton's National Action Network organized the "Justice for Trayvon" rallies and vigils outside federal buildings in at least 101 cities: from New York and Los Angeles to Wichita, Kan., and Birmingham, Ala.
Rallies are scheduled for noon local times. On Saturday morning on a plaza in downtown Manhattan, Sharpton spoke to supporters, telling them he said he wants a rollback of stand-your-ground self-defense laws.
"We are trying to change laws so that this never, ever happens again," Sharpton said.
Martin's mother, Sybrina Fulton, also spoke to the New York crowd. "Today it was my son. Tomorrow it might be yours," she said.
Martin's brother, Jahvaris Fulton, also was to attend the New York event. Martin's father was scheduled to participate in a rally in Miami, where a crowd also gathered Saturday morning.
Sharpton and other supporters want the Justice Department to pursue federal civil rights charges against Zimmerman.
This week, Attorney General Eric Holder announced that the department would investigate whether Zimmerman could be charged under those federal civil rights laws, which would require evidence that he harbored racial animosity against Martin. Most legal experts say that would be a difficult charge to bring.
Holder also said the shooting demonstrates the need to re-examine stand-your-ground laws nationwide.