FACTBOX-Illegal immigration in the United States

July 22 (Reuters) - The Obama administration goes to court on Thursday to argue for a preliminary injunction to block Arizona's law clamping down on illegal immigrants from taking effect on July 29.

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

* There were an estimated 10.8 million illegal immigrants living in the United States on Jan. 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 population of 6.6 million in 2009, including an estimated 460,000 illegal immigrants.

* Arizona, which borders Mexico, is the principal corridor for illegal immigrants entering the United States. The U.S. Border Patrol's Tucson sector arrested about 650 people a day during the first six months of this year.

* Republican Governor Jan Brewer signed the state's immigration bill into law on April 23. It requires state and local police officers to arrest those unable to provide documentation proving they are in the country legally during the course of lawful contact such as a traffic stop. It also makes it a crime to transport someone who is an illegal immigrant, and to hire day laborers off the street.

* The Obama administration has argued that immigration matters are handled on a federal level and that the Arizona law is pre-empted under the U.S. Constitution and federal law. It is seeking a preliminary injunction blocking the law, arguing it would cause irreparable harm if it goes into effect.

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

* 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 Peter Cooney)