secret police

from WordNet (r) 3.0 (2006)

secret police
    n 1: a police force that operates in secrecy (usually against
         persons suspected of treason or sedition)

from Moby Thesaurus II by Grady Ward, 1.0

52 Moby Thesaurus words for "secret police":
      Cheka, FBI, Gestapo, Interpol, MP, MVD, Mounties, NKVD, OGPU, RCMP,
      SP, Scotland Yard, bugging, cloak-and-dagger work, constabulary,
      counterespionage, counterintelligence, county police,
      electronic surveillance, espial, espionage, following,
      highway patrol, intelligence, intelligence work,
      law enforcement agency, military intelligence, military police,
      observation, police, police force, political police, posse,
      provincial police, riot police, secret service, security force,
      shadowing, shore patrol, special police, spying, stakeout,
      state police, surveillance, tactical police, tailing, trailing,
      troopers, vigilance committee, vigilantes, wiretap, wiretapping