It seems to me that cordoning off an entire city with police would be very cost ineffective for a corporation.

How about... instead, a series of automated sentry guns on poles. That would contain most of those within the perimeter. Now, knowing that many of those inside the perimeter are carrying or capable of producing improvised munitions capable of taking out a perimeter gun they would likely have a select group of competent military contractors on standby to contain the perimeter and insert via helicopter.