It's "the police". "Police" is always a plural noun, and you cannot use the article "a" with it.

It's "the police". "Police" is a plural noun, and you cannot use the article "a" with it.

Did you call the police?

Did you call a police?

Remember, plural nouns, like "police", take a plural form of a verb.

The police are investigating a two-vehicle crash.

The police is investigating a two-vehicle crash.

To refer to an individual member of the police force, don't use the phrase "the police" or "a police". You can say "a police officer", "a policeman", or "a policewoman", or "a cop" (slang/informal).

A police officer apprehended the suspect last Friday.

A police apprehended the suspect last Friday.

When using "police" as an adjective, it can be preceded by the indefinite article "a" or another determiner according to the usual rules

He filed a police misconduct lawsuit.

Be aware that we say "contact the police" or "call the police" (not "call police").

She contacted the police to report a crime.

She contacted police to report a crime.

We can also use the expression call the police on someone (not "call the police about someone").

My neighbor called the police on me because I was playing music too loudly at night.

My neighbor called the police about me because I was playing music too loudly at night.