US immigrant rights groups have sharply criticised a bill approved by Arizona's Congress that makes it an offence for a person to lack the proper paperwork.
One group, the National Council of La Raza, said the bill would turn Latinos, regardless of their legal status, into suspects in their own communities.
Supporters of the bill say it will help bring illegal immigration under control in Arizona.
The state is the main entry point for undocumented immigrants into the US.
This latest measure would cement Arizona's reputation as the US state with the toughest immigration laws in the country.
It gives sweeping new powers to the local authorities to determine a person's legal immigrant status - something which has traditionally been the domain of federal immigration.
Immigrant rights groups have said they will mount legal challenges to overturn the measure if it is signed into law, as expected by the state's Republican governor, Jan Brewer.