California became a "sanctuary state" Monday, as a bill that Democratic Gov. Jerry Brown signed into law in October officially took effect.

The law bars police in the nation's most populous state from asking people about their immigration status or participating in federal immigration enforcement activities in most cases, Fox News reported.

The Golden State is home to an estimated 2.3 million illegal immigrants.

“These are uncertain times for undocumented Californians and their families, and this bill strikes a balance that will protect public safety, while bringing a measure of comfort to those families who are now living in fear every day,” Brown said on the day he signed the bill.

But the measure has drawn a rebuttal from an unidentified source, as signs mocking the law have appeared below the "Welcome to California" signs that greet motorists as they enter from Arizona and Nevada.

"OFFICIAL SANCTUARY STATE," the signs declare. "Felons, Illegals and MS13 Welcome! Democrats Need The Votes!"

California passed the bill just as the Trump administration was pledging to crack down on sanctuary cities that refuse to cooperate with federal immigration authorities.

Trump repeatedly pointed to the case of Kate Steinle, a woman who was fatally shot July 1, 2015, on a San Francisco pier by an illegal immigrant. The defendant went on trial for murder, but was acquitted in November 2017.

“No wonder the people of our Country are so angry with Illegal Immigration," the president tweeted after the verdict, which he labeled "disgraceful."

Other members of the Trump administration spoke out against the California proposal.

“The bill risks the safety of good law enforcement officers and the safety of the neighborhoods that need their protection the most,” Attorney General Jeff Sessions said in September, the Los Angeles Times reported.

Added Thomas Homan, acting director of U.S. Immigration and Customs Enforcement: “By passing this bill, California politicians have chosen to prioritize politics over public safety.

“Disturbingly, the legislation serves to codify a dangerous policy that deliberately obstructs our country’s immigration laws and shelters serious criminal alien offenders," Homan's statement continued.

Prior to Brown's approval, California's Legislature passed the measure in September.

The Associated Press contributed to this story.