Even in sanctuary cities, U.S. Immigration and Customs Enforcement officials are free to arrest people they have cause to believe are living in the U.S. without authorization, typically a civil, not criminal, violation.
ICE has a major field office in lower Manhattan, and its officers arrest hundreds of migrants across New York City each year under both Republican and Democratic presidents.
The U.S. Department of Homeland Security can deport non-citizens once an immigration judge has issued a final order of removal.
The main point of contention has been over how some cities handle what ICE calls detainer requests. ICE officials routinely ask local prison, jail or police officials to continue to detain a migrant who was free to leave custody: they have posted bail, a judge has ordered their release, or they have completed a prison sentence.