secret police vs police force

secret police and police force both are nouns.

Word	Noun	Adjective	Verb	Adverb
secret police	Yes	No	No	No
police force	Yes	No	No	No

As nouns, police force is a hypernym of secret police; that is, police force is a word with a broader meaning than secret police:

secret police: a police force that operates in secrecy (usually against persons suspected of treason or sedition)
police force: the force of policemen and officers

Other hypernyms of secret police include constabulary, law, police.

secret police (noun)	police force (noun)
a police force that operates in secrecy (usually against persons suspected of treason or sedition)	the force of policemen and officers