A police force that operates in secrecy (usually against persons suspected of treason or sedition) WordNet 3.0 & Open English WordNet

Secret police are police, intelligence, or security agencies that engage in covert operations against a government's political, religious, or social opponents and dissidents. Wikipedia

A police agency beyond the law to protect the political power of a dictator or regime Wikipedia Disambiguation

Intelligence agency and or police agency, law enforcement office which operates in secrecy Wikidata

A police force operating in secrecy and outside the normal boundaries of law and hidden from the public, usually in support of a totalitarian government's political policies to suppress political dissent through the use of methods including intimidation, violence, and surveillance. Wiktionary

Police operating outside normal boundaries of law. Wiktionary (translation)