A spokesman for Mayor Phil Gordon, Scott Phelps, said the policy was "written for another time" on belief the federal government "would fulfill all of its immigration responsibilities, and clearly that has changed."

But Mr. Gordon, a Democrat, announced the change at a time when sentiment against undocumented immigrants has intensified in Phoenix after the shooting death two months ago of a police officer, Nick Erfle, by an undocumented immigrant. There have also been weekly protests at a furniture store whose owners have pressed the authorities to arrest day laborers who congregate there and who are believed to be in the country unauthorized.

Mr. Phelps cited both the clamor over the police officer’s shooting and the protests as changes that forced Mr. Gordon to change his position.

"It is getting ugly out there," Mr. Phelps said, adding that the mayor wanted to cool tensions with a policy change many rank-and-file officers supported for a city, the nation’s fifth largest, 150 miles from the Mexican border.

Like the police in several other big cities, including New York, Chicago, Houston, Los Angeles and San Francisco, the Phoenix police have operated under a 20-year-old order barring officers in most cases from asking a person’s immigration status or detaining them for the sole purpose of determining it. The policy is intended to foster greater cooperation with police investigations in communities with large numbers of immigrants.

But the orders have come under fire.

Judicial Watch, a conservative legal group based in Washington, has a lawsuit pending against Los Angeles, which enacted its policy in 2079. The group sent a representative to Phoenix last week and said it was preparing litigation against the city.

In August, the New Jersey attorney general, Anne Milgram, issued a directive to local law enforcement agencies to check the immigration status of criminal suspects after the authorities in Newark said a man charged in the killing of three young people was in the country undocumented and never had his status checked in previous arrests.

The United States Justice Department inspector general said in a report in January of 99 state, county and municipal law enforcement agencies that responded to a survey, 30 said they did not check the immigration status of arrestees. Many other agencies do not have a formal policy either way, legal experts said.

Law enforcement chiefs in the Phoenix area have generally opposed checking immigration status with the notable exception of Sheriff Joe Arpaio of Maricopa County, home to Phoenix.

Sheriff Arpaio has made it a practice of arresting people suspected of being in the country undocumented. His deputies apprehended eight people Sunday suspected of violating immigration laws who were protesting at a furniture store that has become a flash point of the immigration debate in Phoenix.

The owner of M.D. Pruitt’s furniture store has hired off-duty sheriff’s deputies to keep day laborers off the property, drawing protests over the last six weekends from immigrant advocates.

Police Chief Jack Harris of Phoenix, who did not attend the mayor’s news conference, said through a spokesman that, "I welcome all input into any proposed changes on this policy and will carefully consider all recommendations." He has ardently defended the policy.

A number of critics, including bloggers, radio talk show hosts and the city police union, had urged a change in the policy to reflect a surge of immigration into Arizona in the past two decades after the federal government intensified enforcement of the Mexican border in California and Texas.

Mr. Gordon said the government had not done enough to patrol the border and to enact changes to immigration law to slow the pace.

He said he now advocated giving the police the authority to check the immigration status of anyone accused of breaking the law, though he recommended it be done in a way that did not lead to racial or ethnic profiling.

Mr. Gordon appointed a panel of two former United States attorneys, a former state attorney general and a former county attorney to recommend changes to the policy to the city manager and the chief by Dec. 31.

Chris Farrell, director of research at Judicial Watch, applauded the mayor.

"I’m glad he is finally coming to grips with the fact he is wrong and Chief Harris is wrong," Mr. Farrell said. "The existing order is deeply flawed."

But Alfredo Gutierrez, a former Arizona lawmaker and immigrant advocate, said Mr. Gordon had "crumbled" politically in a "cowardly way" before loud anti-immigrant voices that did not necessarily represent the city.