CHICAGO - Angered by a controversial Arizona immigration law, tens of thousands of protesters - including 50,000 alone in Los Angeles - rallied in cities across America demanding that President Barack Obama tackle immigration reform immediately.

"I want to thank the Governor of Arizona because she's awakened a sleeping giant," said labour organiser John Delgado, who attended a rally in New York where authorities estimated 6500 gathered.

From Los Angeles to Washington DC, activists, families, students and even politicians marched, practised civil disobedience and "came out" about their citizenship status in the name of rights for immigrants, including the estimated 12 million living illegally in the United States.

Public outcry, particularly among immigrant rights activists, has been building for more than a week since Arizona Governor Jan Brewer signed the legislation. The law requires police to question anyone they suspect of being in the US illegally.

Supporters say the law is necessary because of the federal Government's failure to secure the border, but critics contend it encourages racial profiling and is unconstitutional.

"It's racist," said Donna Sanchez, a 22-year-old US citizen living in Chicago whose parents illegally crossed the Mexican border. "I have papers, but I want to help those who don't."

Organisers estimated about 20,000 gathered at a park on Chicago's West Side and marched, but police said about 8000 turned out.

Obama once promised to tackle immigration reform in his first 100 days, but has pushed back that timetable several times. He said last week that Congress may lack the appetite to take on immigration after going through a tough legislative year.

However, Obama and Congress could address related issues, such as boosting personnel and resources for border security, in upcoming bills.

In Dallas, police estimated at least 20,000 attended a rally.

Among them was Juan Hernandez, the Hispanic outreach co-ordinator for Senator John McCain's unsuccessful presidential run. He said Arizona was once considered by those south of the border to be a model state with particularly close ties to Mexico.

"It went beyond what most states do. Now they are a state that goes beyond what the Constitution says you should do."

About a dozen people at the Dallas event carried signs depicting Brewer as a Nazi, and Maricopa County Sheriff Joe Arpaio, known for his tough illegal immigration stance, as a Klansman. A smattering of counter-protesters showed up at rallies.

In Tucson, Arizona, a few dozen people from the group Arizonans For Immigration Control showed up in support of the new law and Brewer.

- AP