Could a terrorist organisation capture a western city?

We all know a large group of armed men marching down the road towards a city are going to be met by the army and stopped long before they get anywhere near.
But what if those same men are already living in the city? Their weapons smuggled in over a long period of months even years as quietly their manpower grew undetected?

You can't just bomb your own cities to get rid of them and sending the military in risks huge civilian casualties.

So just how many men do you need to capture and hold a western city?
Could a terrorist organisation capture a western city?
Post Opinion