With as many as 100,000 people expected to convene downtown Saturday to rally in support of immigrant rights, Los Angeles Police Chief Charlie Beck went out of his way to make clear that his officers are guided by a different set of rules than the controversial state law recently passed in neighboring Arizona.

"Because of the ongoing issues in Arizona, I wanted to make something perfectly clear," Beck said. "Special Order 40 is the mandate of this department. That is how I will police this city as long as I am the chief of police."

Beck was referring to the agency's long-standing policy that prohibits officers from initiating contact with someone solely to determine whether he or she is in the country legally. It has drawn the periodic ire of critics who argue for a tougher stance against undocumented immigrants.

Beck and other chiefs before him, however, have said the policy is an important tool, as it encourages illegal immigrants who witness crimes to assist police without fear of being deported.

Arizona's tough new legislation makes it a state crime to be in Arizona illegally and requires police to check suspects for immigration paperwork.

The legislation also bars people from soliciting work or hiring day laborers off the street.

Anger over the law has inflamed passions on both sides of the immigration debate and caused LAPD officials to increase their predictions for the number of people expected to turnout for the annual demonstration.