Philadelphia Mayor Michael Nutter says the city will limit its cooperation with federal immigration officials.
The mayor signed an executive order on the matter yesterday.
The city's police department had previously complied with federal requests to keep immigrants charged with minor crimes detained longer than normal.
Under the new order, those charged would be released pending trial.
Nutter says too many people were ending up in deportation proceedings for only minor infractions.
Opponents say the change will make the city less safe.