I would always use "police" in the plural, even if it's as a single public institution. "The police are here to serve and protect". I can't think of ever saying "the police is...". That would just sound wrong to me. Perhaps some of the confusion about "police" is that normally this is a case where there's a difference in collective nouns between American and UK English. For example the choice between "the government is" and "the government are". Supposedly Americans would prefer "the government is", but I'm not American so I couldn't say for sure. I had to check to make sure, but it would appear that "police" is a special exception in English, and supposedly uses the plural in all forms of English. When talking about the police as individual people, I'd personally hesitate to say something like "one hundred police", and say "one hundred police officers" instead. I can't personally say that "one hundred police" is wrong, but it does sound a little awkward to me. That may be a style difference. However, "one hundred cops" is another informal way to say it. Coincidentally, I still run across interesting advice on this site from time to time: https://www.btb.termiumplus.gc.ca/tpv2guides/guides/pep/index-fra.html?lang=fra&page=grammar_8_grammar_numbers To quote it: "Other collective nouns Collective nouns that are less number-oriented can be equally troublesome. Group, team, committee, staff and so on—are they singular or plural? Here, usage authorities take slightly different positions depending on which side of the Atlantic they call home. In the U.K. these nouns are usually treated as plural. In North America they’re usually treated as singular, except when the members of the collective are acting independently, in which case the nouns are considered plural."

"Police" as a public institution is usually just referred to as "The Police". Depending on where you are the police could also have an extra title as an institution. Examples: England - The Police Force America - The Police Department In an informal setting however, there's usually a local colloquialism that's used instead. Example: America - The Cops Hope this helps.

'police' can be a singular noun and a plural noun, as well as a verb. Note that it can be a countable noun as in this example: "More than one hundred police have ringed the area." I hope this helps, Richard