Only the first sounds natural. "The police" is plural.
You could say: "The police force has become alert."
po·lice /pəˈlis/ Show Spelled [puh-lees] Show IPA noun, verb, -liced, -lic·ing.
–noun
1. Also called
police force. an organized civil force for maintaining order, preventing and detecting crime, and enforcing the laws.
2. ( used with a plural verb ) members of such a force: Several police are patrolling the neighborhood.
3. the regulation and control of a community, esp. for the maintenance of public order, safety, health, morals, etc.
4. the
department of the government concerned with this, esp. with the maintenance of order.
5. any
body of people officially maintained or employed to keep order, enforce regulations, etc.
Police | Define Police at Dictionary.com