Es geht beides, je nachdem, ob man an die "Polizei" als Institution oder an die einzelnen Polizisten denkt. Es gibt Präferenzunterschiede zwischen AmE und BrE.

Nomen wie police nennt man auch "group nouns" oder "collective nouns". Googel das mal oder schau im Leo-Archiv, es wurde schon sehr oft diskutiert.

Always use a plural verb with "police".

My (BE) feeling is that you say "the police are", because you're considering police officers as a group. The police force is..., the constabulary is...

http://www.cambs.police.uk/about/ or http://www.thamesvalley.police.uk/aboutus.htm give various usages. I've just noticed that "Thames Valley Police is..." is used, but then it goes on to talk about "we" and "our". So maybe "Police is" where it's a specific title, and could equally well be "Police Force". But I think I'd say "police are" when it's being used as short for "police officers".

Whatever you do, be consistent.

See for example:

http://www.oup.com/uk/booksites/content/01992...
Most collective nouns can be treated as singular or plural, with either a singular or plural verb:

The whole family was at the table. [singular verb]
The whole family were at the table. [plural verb]

But there are a few collective nouns, such as police, which are always used with a plural verb:

She's happy with the way the police have handled the case. [not...the police has handled the case]

TIP: look up collective nouns in the dictionary if you're not sure whether to treat them as singular or plural. Special cases such as police have 'treated as pl.' in brackets before the definitions.

I agree that 'police' is virtually always plural in English: the police are. The very few exceptions would probably be when the word is part of a proper name of a particular police force.

Some collective nouns can be either singular or plural, as in #1-2, but 'police' is not in that group.