New York City has long been known as a refuge that celebrates immigrants, whether they are here legally or not. But some worry that a new federal policy beginning tomorrow will turn the city’s atmosphere from friendly to frightening.

The U.S. government is mandating a program called Secure Communities be enforced in the city starting Tuesday, May 15. Secure Communities requires police to send fingerprints of everyone arrested to immigration officials. Immigration and Customs Enforcement (ICE) agents then check to see whether that person should be deported.

City officials said yesterday the program threatens New York’s immigrant-friendly nature, where police trust immigrants to report crimes, not fear being stopped by cops.

“Secure Communities would put us all at risk by making law-abiding New Yorkers less likely to report a crime or come forward as a witness,” Public Advocate Bill de Blasio said.

Both the city and state fought Secure Communities; in 2011, when the program was optional, Gov. Andrew Cuomo suspended state participation.

But now, it’s mandatory and New York police have no choice but to comply.

Federal officials argue the program is an effective way to deport violent criminals.

According to the Department of Homeland Security, more than 135,000 illegal immigrants convicted of crimes across the nation were removed through Secure Communities since the inception of the program in 2008. Nearly 50,000 of those deported were convicted of offenses like murder, rape and sexual abuse of children.

But critics say that the program does not differentiate between dangerous criminals and people who are arrested during a mere traffic stop. Also at risk for deportation could be those people who are arrested and charged with a crime, only to have the charges dropped later.

Some also worry the program will make immigrants less likely to trust police and report crimes, such as terrorism, in their neighborhoods.

De Blasio penned a letter to U.S. Attorney General Eric Holder Sunday, asking him to halt the program. And City Council Speaker Christine Quinn said she would monitor the program and is drafting legislation to clarify the limits of NYPD contact with immigrants.

What does the NYPD do now?

Right now, when ICE asks New York to hold a prisoner, the city only honors the request if the person meets one of the following criteria:

Previously convicted of a misdemeanor or felony
Defendant in a pending criminal case
Has an outstanding criminal warrant
Is or has been subject to a final deportation order
Is a known gang member in the FBI’s national gang database
Identified as a possible match in terrorist screening database

“We wish they would have looked at the process we developed here, which strikes the right balance by protecting public safety and national security while ensuring we remain immigrant friendly,” said John Feinblatt, a chief policy advisor to Bloomberg.

How does the process work?

An ICE spokesman said that the fingerprints are checked against their database of immigrants who entered the country legally, looking for whether arrestees are listed there.

And that database might also include immigrants who came into the country legally, but overstayed their visas or were picked up by the police but let go while immigration proceedings continued, an ICE spokesman said.

If the fingerprints do not match at all, officials investigate things like Social Security numbers to find out whether the person might be in the country illegally. Read more from Metro New York