More than 100,000 people packed the National Mall in Washington; at least 75,000 marched through the Arizona city of Phoenix; tens of thousands packed the streets around City Hall in New York and more than 30,000 paraded through Atlanta, Georgia.
Another giant rally gathered in Los Angeles and smaller rallies were held in more than 100 cities across the US, in a bid to put pressure on Congress over immigration reforms.
The US House of Representatives has passed legislation that would make illegal entry a crime and step up the construction of a wall on the US-Mexico border.
Efforts in the Senate to agree a compromise bill, which could open the way for many undocumented workers to be legalised, collapsed last week.
In Washington, marchers waving US flags chanted “USA, USA” and “Si se puede” (Yes we can), as speakers including Senator Ted Kennedy and other politicians and religious leaders encouraged them to fight for citizenship.
“This debate goes to the heart of who we are as Americans,” Senator Kennedy said to cheers, as his words were translated into Spanish.
“It’s about good people who come to America to work,” he said.
Mexican Joel Nieto, 43, said the marches highlight immigrants’ unity as well as their worries.
“We aren’t delinquents or bad people, we are cheap manual labor. They do not want to give papers to us, but they need our work,” said Mr Nieto, who has worked numerous jobs since arriving 10 years ago.
The new wave of demonstrations have been building again since the Senate
On Sunday, up to 500,000 people marched through Dallas, Texas demanding a legal status for undocumented workers, who authorities admit do the low-paid “dirty” jobs that many Americans don’t want to do.
Protests also surfaced in small towns around the country.
Around 1,500 hit the streets in Jackson, Mississippi, and up to 4,500 gathered in Lake Worth, Florida.
Laura Hannah, spokeswoman for Lake Worth’s mayor, called it the “biggest protest in recent times.”
“This is a national movement of justice for the immigrants, the fight for civil rights of our generation,” Joshua Hoyt, director of the Illinois Coalition for Immigrant and Refugee Rights, told AFP.
“The political parties have to understand that this is something that is not going to disappear,” Mr Hoyt said in Chicago.
The immigration problem is a growing concern for mainstream America ahead of November’s mid-term elections.
Three-quarters of Americans believe the government is not doing enough to stop illegal immigration, according to a poll released on Monday by The Washington Post and ABC television.
On Monday, US President George W Bush, whose proposed “guest worker” programme for immigrants has failed to overcome a major split in his own Republican Party, called both for compassion and for enforcing the laws over immigration.
“Understand that we’re a nation of immigrants, that we ought to be compassionate about this debate,” he said, adding: “Obviously, we’ve got to secure the border and enforce the law.”
A majority of the illegal workers in the US are from Mexico and other Latin American countries.
Berta Sanchez, a Salvadoran in a Washington suburb who gained legal status after working in the country nearly two decades illegally, headed for the rally in the capital with her husband and two children.
“I am going to support my people. We also did not have papers and it was very difficult. We want to show that we are united, whether we have papers or not,” she told AFP.
Construction worker Manuel Cruz, an undocumented immigrant in the Washington region for 11 years, said he wanted legal status.
“The work we do is heavy, most Americans do not want to do it, they prefer to sit at their computers,” said his wife Maria Rosa.
Organisers pointed out that many of the protestors are legal immigrants with voting rights.
“We march in the streets, but we will also march to the voting booth in November,” said Eliseo Medina, head of the Service Employees International Union, an organiser of the New York march.