This is in chapter 4 mission 2. As you're moving throughout the level there will be men in dark brown outfits carrying spears, these are the civilians. Some of them will die at the start, but they do not count for the achievement.
After you've saved the civilian lying wounded next to the obelisk statue (the guy that starts to thank you, then says you won't be forgiven for your crimes) you must break a gate and move into a circular area with a fountain. All around are civilians being harassed by templars, these are the guys you have to save.
If you want to play this part safe, don't rush in. If you stay away from the templars they'll just harass civilians, they only begin to attack when you get near. Use Roaring Flames (LT + X) to long range burn the templars, the civilians will flee while the attacker is on fire so there's no chance of them being hurt. It's possible that two templars will be on a civilian, you can use Maelstrom of Fire (LT + B) instead to stun them both.
There's another area right after pushing the cart out of your way with 3 more civilians to save, one of which is right near the cart so you might want to be quick. Once the 3rd guy has been saved the achievement will pop.