The two-mile Atlanta march was in support of immigrant rights nationally as well as in protest of state legislation awaiting Gov. Sonny Perdue's signature. If signed, it would require that adults seeking many state-administered benefits prove they are in the country legally.
Carlos Carrera, a construction worker from Mexico, held a large banner that read: "We are not criminals. Give us a chance for a better life."
"We would like them to let us work with dignity. We want to progress along with this country," Carrera said. He said he had been in the United States for 20 years.
In Pittsburgh, a smaller group marched to Sen. Arlen Specter's office.
"We all know pay is not the same everywhere and lot of people won't work for the minimum here, so if they won't take the job, what's the problem?" said 47-year-old Jose Salazar.
Similar marches were planned for New York, Washington, D.C, and dozens of other cities.
Monday's demonstrations followed a day of rallies in 10 states that drew an estimated 350,000 to 500,000 in Dallas alone.
In Salt Lake City, 20,000 turned out on Sunday, far more than expected, police said, and 50,000 rallied in San Diego. Other demonstrations were held in Minnesota, New Mexico, Michigan, Iowa, Alabama, Oregon and Idaho.
With an overhaul of immigration law stalled in Congress, demonstrators urged lawmakers to help an estimated 11 million illegal immigrants settle legally in the United States.
"If we don't protest they'll never hear us," said Oscar Cruz, 23, a construction worker who marched in San Diego. Cruz, who came illegally to the U.S. in 2003, said he had feared a crackdown but felt emboldened by the large marches across the country in recent weeks.
In Birmingham, Ala., demonstrators marched along the same streets where civil rights activists clashed with police in the 1960s and rallied at a park where a statue of the Rev. Martin Luther King Jr. stands as a reminder of the fight for equal rights and the violence that once plagued the city.
"We've got to get back in touch with the civil rights movement, because that's what this is about," said the Rev. Lawton Higgs, a United Methodist minister and activist.
In Salt Lake City, Jerry Owens, 59, a Navy veteran from Midway wearing a blue Minuteman T-shirt and camouflage pants, held a yellow "Don't Tread on Me" flag.
"I think it's real sad because these people are really saying it's OK to be illegal aliens," Owens said. "What Americans are saying is 'Yes, come here. But come here legally.' And I think that's the big problem."
Many groups had been preparing to rally since December, when the House passed a bill to build more walls along the U.S.-Mexico border, make it a crime to help undocumented immigrants and make it a felony to be in the country illegally. It is now a civil violation.
Since then, local and regional protests, supported by popular Spanish-language disc jockeys, quickly merged into national plans after hundreds of thousands of people demonstrated in dozens of cities last month, culminating March 25 with a 500,000-strong rally in Los Angeles.
"This is a force, an energy here," said Amir Krummell, a U.S. citizen born in Panama, who marched to Dallas' city hall on Sunday. All around him in the wave of protesters could be heard shouts of "Si Se Puede!", Spanish for "Yes, we can!"
Public opinion sides with the illegal immigrants, reports CBS News correspondent Alison Harmelin. In a new CBS News poll, 74 percent of Americans feel they should be able to stay in the U.S. if certain criteria are met. That includes:
But critics say it's unfair to allow immigrants who have sneaked into the country to be given preference over those waiting on a list to get citizenship.
And while the rallies unfold, Congress is in recess after failing to reach a compromise last week. But the chairman of the Senate Judiciary Committee is promising to have a new bill ready for debate when Congress returns from Spring break.
"I think tempers will cool over a two-week period," said Sen. Arlen Specter, R-Pa. "And also, there are going to be some expressions by many people very unhappy with the Senate not passing a bill and very unhappy with the House bill" that would make being an illegal immigrant a felony.
"There's a real risk of significant political fallout here, and members of the Senate think about that, believe it or not," Specter said on "Fox News Sunday."