Carrying Mexican flags and shouting defiance, streams of Hispanic teenagers have been pouring out of high schools and onto the streets in protest at the measure which passed by a vote of 59 per cent to 41 per cent.
The most wide-ranging measure against illegal immigration ever undertaken in the United States, Proposition 187 would deny illegal immigrants access to virtually all public services, including education.
It would require public officials to turn in anyone they suspect of being in the country illegally.
Headteachers in Southern California have denounced the measure as racist and announced their refusal to ask students for documentation.
Los Angeles has said it will not implement it, and a barrage of lawsuits is hitting the courts to block its taking effect.
But as civil rights groups, ethnic minority organisations and local government bodies went to court, Governor Pete Wilson, who made support for the controversial initiative a central theme of his campaign, ordered the state government to begin to enforce the measure.
It denies welfare services, non-emergency medical treatment and public schooling to foreigners living in the state illegally.
However, a federal judge barred enforcement of the measure for one week, until he can hold a hearing. And a state judge in San Francisco ruled that state officials cannot expel illegal immigrants from public schools until the legal controversy is resolved.
There are estimated to be almost 1.5 million illegal immigrants in California, of whom 300,000 are believed to be children in school.
Governor Wilson said the state would fight its case all the way to the Supreme Court.
As he was instructing state officials to enforce the new law, he said headteachers and others who refused to ask studentsfor documentation should be fired.
In a survey, the Los Angeles Times found voters deeply divided on the issue along racial lines.
Whites were broadly in favour; ethnic minorities against. Cardinal Roger Mahoney, Roman Catholic archbishop of Los Angeles, issued a statement saying the vote "added another sad chapter to the history of California".
Mexicans also condemned the result. President Carlos Salinas de Gortari issued a statement which said: "What will happen to the children denied schooling? Will they return to Mexico, wash windshields in California, sell newspapers on the streets or beg?" Elsa R Saxod, executive director of the United States-Mexico Border Progress Foundation, said: "All of the talk was that this vote was meant to send a strong message to Washington, but what about the very mean-spirited sort of message it sends across the border?"