What is a sanctuary city and how might those cities be affected by President Trump's executive order? On Wednesday, Mr. Trump signed an executive order that would withhold federal grant money from sanctuary cities.

Sanctuary cities have policies against turning over undocumented immigrants who might otherwise be deported by federal immigration law enforcement officials. There are over 140 sanctuary jurisdictions -- cities and counties -- across the U.S., including at least 37 cities -- San Francisco, New York, Chicago, Seattle, Miami and Los Angeles, among others.

Boston Mayor Marty Walsh is promising to let immigrants who feel threatened take shelter in City Hall if need be.

Moina Shaiq holds a sign at a rally outside of City Hall in San Francisco, Wed., Jan. 25, 2017. President Donald Trump moved aggressively to tighten the nation's immigration controls Wednesday, signing executive actions to jumpstart construction of his promised U.S.-Mexico border wall and cut federal grants for immigrant-protecting. AP

"Jurisdictions that willfully refuse to comply" with federal immigration laws, the order says, "are not eligible to receive Federal grants, except as deemed necessary for law enforcement purposes by the attorney general or the secretary."

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Many mayors, including Chicago Mayor Rahm Emanuel, said they'd defy the order.

"We're going to stay a sanctuary city," Emanuel said, according to the Chicago Tribune. "Wherever you came from, you're welcome here."

The mayors of the Bay Area's three largest cities, Oakland, San Francisco and San Jose, and the City of Berkeley spoke out against President Trump's executive order on immigration.

Berkeley Mayor Jesse Arreguín, San Francisco Mayor Ed Lee, San Jose Mayor Sam Liccardo and Oakland Mayor Libby Schaaf also vowed to take a regional approach to combat the impacts of any threatened cuts in federal funding.

But a day after Mr. Trump signed the order, Miami-Dade Mayor Carlos Gimenez ordered county jails to begin complying with federal immigration detention demands, becoming the first mayor to abandon the sanctuary status over the fear that his city would lose millions in federal funds.

The order does not specify how much or what kind of funding would be or could be blocked, although White House press secretary Sean Spicer said Wednesday that the homeland security secretary would "look at funding streams that are going to those cities and look at how we can defund those streams."

CBS News' Carter Evans spoke with Pedro Trujillo, whose undocumented parents brought him to the U.S. when he was 7.

"There's anxiety going around, here's a lot of worry ... Are there going to be raids coming our way in the coming months? We don't know that yet," Trujillo said

In 2016, a Justice Department inspector general's report investigated how much in Justice Department federal grants some sanctuary jurisdictions receive (as of Mar. 2016). Over 60 percent of the funding goes to 10 jurisdictions identified by the report: