Factbox: Illegal immigration in the United States

(Reuters) - Thousands of Hispanic, religious and labor activists are expected to join May Day rallies in cities across the United States on Saturday, protesting Arizona's tough crackdown on illegal immigrants and urging President Barack Obama to overhaul federal immigration laws.

Here are some facts about illegal immigrants in the United States, together with details of the controversial Arizona state law, reform initiatives backed by Obama, and measures enacted by other states:

* There were an estimated 10.8 million illegal immigrants living in the United States on January 1, 2009.

* Most were from Latin America, with some 6.7 million from Mexico and 1.33 million from Central American nations El Salvador, Guatemala and Honduras.

* Arizona had a total population of 6.6 million in 2009, including an estimated 460,000 undocumented immigrants.

* Arizona, which borders Mexico, is the principal corridor for illegal immigrants entering the United States. The U.S. Border Patrol's Tucson sector has made an average of 650 arrests a day this year.

* President Obama backs a comprehensive overhaul of federal immigration laws. Any reform bill has a slim chance of passage in Congress this year. Obama has said lawmakers may not have the appetite to tackle it ahead of the November elections. But some Democrats fear delay could cause a backlash among Latino voters.

* Obama supports a system that allows undocumented immigrants in good standing to pay a fine, learn English and become citizens. He also backs tightening border security and clamping down on employers that hire undocumented workers.

* The last attempt to overhaul the U.S. immigration system, by his predecessor, President George W. Bush in 2007, was torpedoed by Republicans in Congress.

* Arizona's law is the toughest, but by no means the only, immigration-related measure passed by U.S. states, which traditionally leave border security to the federal government.

* Under Arizona's new law, state and local police officers are required to arrest those unable to provide documentation proving they are in the country legally. It also makes it a crime to transport someone who is an illegal immigrant, and to hire day laborers off the street.

* In the first three months of this year, more than 1,180 bills and resolutions relating to immigrants and refugees were introduced in state legislatures across the country. Of those bills, 71 laws were enacted and 87 resolutions adopted in 25 states.

Sources: U.S. Department of Homeland Security, U.S. Census Bureau and the National Conference of State Legislatures.

(Reporting by Tim Gaynor; editing by Stacey Joyce)