Here we can see that the term "police" is listed as a special case among plural forms of nouns.
The following nouns are conceived as plural rather than singular and so have only the plural form:
headquarters goods clothes premises means police people
e.g. The police are looking for larger premises to build the new headquarters.
It appears in this Wiki article that others understand "police" as an uncountable.
SOME UNCOUNTABLE NOUNS ARE PLURAL. These have no singular form. e.g. clothes, groceries, thanks, jeans, police, trousers, scissors.
There are some collective nouns which have singular forms but are followed by plural verbs. This mean they have singular forms but have plural sense. For example police, people, military, cattle.
Here's another reference that considers "police" to be uncountable.
An organisation granted the legal authority to enforce the law. See usage note.
Compared with the uncountable singular nouns, the family of uncountable plural nouns is relatively small. Here are 10 more members:
- Trousers (are)
- The police
Here's another example of "police" referred to as an uncountable noun.
police listed as uncountable
The Longman Dictionary of Contemporary English provides an interesting instrucion for dealing with the word "police", and it's what I would say as well. It's interesting that we can find examples of "two police" and "three police", yet the LDOC says don't say "a police", which means "one police".
! Police is a plural noun. Do not say 'a police'. Say a police officer, a policeman, or a policewoman: The police were called. | A police officer came. ➔ military police, secret police