Thousands of people have protested outside the US Congress against an immigration bill ahead of a Senate debate on the controversial proposals.
Demonstrators demanded that all illegal immigrants be granted residency.
Research released on Tuesday suggests there are 11.5 million undocumented migrants in the US.
The Pew Hispanic Center says illegal immigration has been rising steadily since 2000, despite increased security on the US-Mexico border.
More than half the undocumented immigrants come from Mexico, but there have been large increases in illegal migration from South and East Asia, the study says.
UNDOCUMENTED WORK FORCE
In certain occupations, such as farming, cleaning, construction and food preparation, they are reported to account for a large proportion of the work force.
The report is based on a March 2005 population survey.
'Shameful'
The controversial bill which is being debated by the Senate on Wednesday includes proposals to use of troops and police to halt migrants, and tighter employment controls.
The bill - which has already been passed by the House of Representatives - also proposes the construction of a 1,130-km (700-mile) fence on the Mexican border.
Washington has accused Mexico of not doing enough to stem the flow of illegal immigrants into the US.
But Mexican President Vicente Fox has described the fence plan as "shameful".
He recently told the BBC in 10 years' time the US would have to "beg for Mexican workers" to cover its needs.
An estimated 1,500 Latin Americans, mostly Mexicans, are believed to cross into the US illegally every day.
President George W Bush has said that "border security must adapt to the nation's changing needs".
^ Back to top | BBC Sport Home | BBC Homepage | Contact us | Help | ©