Use the third person, plural form 'are' when referring to police in general. The word police is an aggregate noun, a word representing an indefinite number of parts; aggregate nouns have no singular form.
When referring to a single person or a specific department, use the third person, singular 'is'. Examples:
The police are blocking off the street where the accident occurred.
A police officer is getting information from the neighbors.
The police department is at the corner of First and Main streets.