Just to extend on this - this is how it's done in the UK.
Specifically in London the regular police are not armed. Armed police patrol the city in cars - if you're in the city you can identify these cars as they have a big yellow spot on the windows and there is an officer in the back seat. The aim is to be able to respond to a request within 8 minutes.
During the London bridge attack the attackers were shot dead by armed police 9 minutes after the first call to the police.
Specifically in London the regular police are not armed. Armed police patrol the city in cars - if you're in the city you can identify these cars as they have a big yellow spot on the windows and there is an officer in the back seat. The aim is to be able to respond to a request within 8 minutes.
During the London bridge attack the attackers were shot dead by armed police 9 minutes after the first call to the police.