Hola Mags, when the collective word ("la policía" in this case) is followed immediately by a verb, the noun is treated as singular and therefore the verb must be conjugated in the third person singular. However, if the group entity is followed by other words as the preposition "de", there is no consistency in the language and this means that you can choose any number. For example, you can say "un grupo de personas está en la calle" with the verb in the singular form, and also "un grupo de personas están en la calle", with the verb conjugated in the plural form. Both of them are considered correct as it doesn't exist any particular rule about this grammar point. Thank you very much for your question!