At an abandoned movie ­theater on the Boston-Brookline line, gunmen and their hostages shuffled out the front door, huddled under a thick blanket. Waiting SWAT teams, unable to tell friend from foe, could not shoot, but had to act quickly, or the group would make it to a getaway van.

That was one of the scenarios Boston-area first responders faced Saturday as part of an inten­sive training exercise dubbed Urban Shield: Boston.