The authorities have arrested a man who law enforcement officials believe was planning to build and detonate a bomb in New York with government workers as the target, a person briefed on the case said on Sunday.

Mayor Michael R. Bloomberg, Commissioner Raymond W. Kelly of the New York Police Department, and Manhattan District Attorney Cyrus R. Vance Jr.were expected to announce the charges against the man at 7:30 p.m. at a news conference at City Hall.