The Rev. Al Sharpton told the crowd the law violates civil and human rights and will lead to racial profiling of U.S. citizens and others who are in the country legally.
"We're going to stop it here before it goes any further to other communities," the New York-based minister and civil rights activist said. "We cannot have a nation where, based on your language or your race, determines your rights. Your rights must be determined by the fact that we're all equal."
The law's author, state Rep. Matt Ramsey, has dismissed concerns about racial profiling, saying the law includes language to protect against it. The Peachtree City Republican and other supporters say the law is needed to keep illegal immigrants from putting a strain on the state's resources, such as hospitals and schools.
The crowd at the Capitol was a sea of green T-shirts worn by members of the American Federation of State, County and Municipal Employees. The union is holding a young workers conference in Atlanta and sent busloads of people to the rally.
"We're still a country that believes all people should benefit from human rights," said union secretary-treasurer Lee Saunders. "I think we've got to make our voices heard on the assault on workers, the assault on immigrants across this country."
Ben Speight, a local Teamsters organizer, echoed those sentiments and said labor groups must get involved.
"Let's get in the way of hate. Let's build a social movement," he said, to loud cheers. "Labor cannot be isolated. We've got to reach out to the community and stand up against hate."
A federal judge has temporarily blocked two parts of the Georgia law until a lawsuit challenging its constitutionality is resolved. State officials plan to appeal the judge's ruling.
One blocked section authorizes police to check the immigration status of suspects who don't have proper identification and to detain illegal immigrants. The other creates a state penalty for people who knowingly and willingly transport or harbor illegal immigrants while committing another crime.
Other parts of the law took effect July 1, including a provision that makes it a felony to use false information or documentation when applying for a job and another that creates an immigration review board to investigate complaints about government officials not complying with state laws related to illegal immigration.
Starting Jan. 1, businesses with 500 or more employees must use a federal database to check the immigration status of new hires. That requirement will be phased in, applying to all businesses with more than 10 employees by July 2013. Also starting Jan. 1, applicants for public benefits must provide at least one state or federally issued "secure and verifiable" document.