In practice, the mayor said, many people have ended up in deportation proceedings after having committed only misdemeanors.
"As a result of overly aggressive use of these detainers," he said, "there has been a negative impact on some immigrants who will not report crimes to the police, don't want to be witnesses, and suffer accordingly."
About 17 localities, including Miami, San Francisco, Newark, N.J., New Orleans, and New York City, have modified their policies on ICE holds.
But where some apply only to police, Philadelphia's new rules apply both to its police and prison departments. The order also means the city won't tell ICE about a prisoner's pending release unless the person was convicted of a violent felony and ICE's request is supported by a warrant from a judge.
"Philadelphia's policy is in fact unique and cutting-edge," largely because of the judicial warrant requirement, said Sunita Patel, a staff attorney with the Center for Constitutional Rights, a nonprofit legal education organization in New York.
Molly Tack-Hooper, a staff attorney with the American Civil Liberties Union of Pennsylvania, said in an interview that Nutter's order means Philadelphia "will no longer detain people just so federal authorities can investigate their immigration status."
Nutter said his administration had always placed limitations on the city's agreement with ICE out of concern that a too-close collaboration could undermine public safety if immigrants came to fear cooperating with local police.
"Today's executive order," he said, "takes our position one step further."
Nicole Navas, a spokeswoman for ICE, said the agency "places detainers . . . to ensure that dangerous individuals are not released from prisons and jails into our communities."
In a separate statement, her office said it would continue to lodge detainers even if Philadelphia ignores those requests. For the city and its surrounding counties, ICE statistics show, detainer requests had already been falling steadily for the last three fiscal years: from 823 in 2011 to 735 in 2012 and 587 in 2013.
Councilwoman Maria Quiñones Sánchez, a proponent of the policy shift, praised the city's diverse advocacy groups and various faith communities. Present for the signing were representatives of Juntos, 1Love Movement, New Sanctuary Movement, HIAS, and the Pennsylvania Immigration and Citizenship Coalition, among others.
"Thank you," said Quiñones Sánchez, "for taking us to task."