Mayor Eric Adams announced the start of a contentious new plan to put a gun-detecting scanner in the subway on Friday and warned that New Yorkers who refused to be scanned would be kicked out of the system.

Mr. Adams highlighted the scanner inside the busy Fulton Street subway station in Lower Manhattan, arguing that the technology would help make the subway safer.

“Many New Yorkers will be familiar with this type of technology — it’s not new, and it’s being used in big cities across the country,” Mr. Adams said, noting that scanners were already being used at the Metropolitan Museum of Art and Citi Field.

A new pilot program will involve one scanner that will be moved to different stations over the next 30 days, city officials said. Mr. Adams said it would not cost the city any money.

Civil rights groups immediately called the scanners unconstitutional and said they were preparing to take legal action.

“New Yorkers did not consent to give up their rights or be N.Y.P.D. guinea pigs for overhyped and error-prone surveillance tech,” said a joint statement from the Legal Aid Society and the New York Civil Liberties Union. “We are prepared to protect the right of all subway riders to be free from N.Y.P.D. intrusion and harassment.”