Boston police Commissioner William B. Evans on Friday joined a legal effort to protect young immigrants from the Trump administration’s effort to abolish a federal policy that has shielded approximately 800,000 people from deportation.

“Simply put, our law enforcement agencies must serve and protect everyone in our communities, and without the mutual trust between police and the immigrants in our cities, we can’t possibly provide the type of safety and protection the communities deserve,’’ Evans said in a statement Saturday about his decision to sign a brief in support of the policy. “We are not proposing protecting people who are here illegally and have committed violent crimes; upholding [the policy] will benefit the law-abiding, hard-working immigrants who contribute to our society.’’

The brief was filed in federal court in Washington, D.C., Friday in support of a federal lawsuit filed last month by the trustees of Princeton University, Microsoft, and an immigrant Princeton student against the United States of America, the US Department of Homeland Security, and the acting secretary of DHS.

The suit seeks to preserve the Deferred Action for Childhood Arrivals policy, or DACA, which allowed certain young immigrants, called “Dreamers,’’ who were brought to the country illegally as children the chance to obtain two-year, renewable work permits — granting them a reprieve from deportation and allowing them to work.

Attorney General Jeff Sessions announced in early September that DACA was being revoked because it was “an unconstitutional exercise of authority,’’ enacted without congressional approval by former president Barack Obama in 2012.

“Part of the current rhetoric coming from the federal government, particularly the attorney general and the Justice Department, is that cracking down on programs like DACA . . . is somehow supposed to be a public safety boost,’’ said Joshua Geltzer, executive director of Georgetown Law’s Institute for Constitutional Advocacy and Protection, who ­coauthored the brief with a Chicago law firm representing the signatories. “That’s just not what the studies show, and it’s not what the people who really do that job —rather than engage in politics talking about doing that job — say.’’

In the brief, Evans and other law enforcement officials argued that the DACA program allowed immigrants to feel safe reporting crimes and testifying as witnesses because they no longer fear deportation.

The brief cites one statistic that showed that nearly two-thirds of Dreamers reported being less afraid of law enforcement, and 59 percent indicated they would report a crime with the DACA protections that they would not have otherwise.

When immigrants are afraid to report crimes, the brief argues, crimes like domestic violence and gang violence do not simply go unpunished — they multiply because “unresolved resentments grow in the community.’’

Undocumented immigrants are already at higher risk for crimes like robbery because they often do not use banks and instead carry cash, or wage theft because they have no legal recourse, the brief contends.

And DACA helps law enforcement do its job more efficiently, the brief argues. When crimes are properly reported, resources can be properly apportioned. DACA also gives immigrants a form of identification, which means that traffic stops and witness interviews don’t get bogged down in a lengthy process of trying to determine a person’s identity — which can often require fingerprinting.

The brief points to the success of the Violence Against Women Act of 2000 as support for maintaining DACA. The act created a U-visa, granting immigration relief to undocumented victims of certain crimes. The brief says a recent study showed that more than 99 percent of U-visa holders said they were willing to cooperate with the police, and 70 percent were asked to provide assistance to police investigating crimes committed against them and did so.

“Community policing works when residents feel like they are full members of our community, where trust in local law enforcement leads to reporting crimes and cooperating in investigations,’’ said Nicole Caravella, a spokeswoman for Mayor Martin J. Walsh, in a statement. “Commissioner Evans recognizes this as key to the Boston Police Department’s goal of maintaining public safety for all who live, work, and visit our city.’’

Michael Levenson of the Globe Staff contributed to this report. Evan Allen can be reached at evan.allen@globe.com. Follow her on Twitter @evanmallen.