For our anti-terrorism courses we have to write up a scenario where were the terrorists, and come up with a plot to attack a target, plan it, and present it.
Mine was simple, my group of 4 jihadis would enter the target location, pull the fire alarm and open fire with guns that are common to acquire, once the targets assembled in the meeting place.
I won't go into further detail as to not give anyone ideas.
But the point of the exercise was this.
How do you prevent it?
Answer: you can't.
What is the best response to this?
Allow the targets to be armed if the law allows them to own firearms. Hell, run your own training course if necessary to prove proficiency, but in the end, allowing the targets to fight back is the only solution, short of hiring and having an armed response team on standby.
Otherwise, never allow a congregation of people as it provides too much of a target.
Those last 2 are almost impossible to come by.
The point is, disarming people always served to make them victims.
I was asked, 'what if you get shot, shooting back at the terrorists?'
I responded, 'well then, I'd be killed fighting terrorists, instead of being a victim of terrorism.'
Arbitrary enforcement of the law is the mark of tyranny. Citizens will always fight against it.