During your travels in LA you will receive a distress call from a woman in Santa Monica. Don't worry this is not timed, you can do it right away, or finish up whatever you were doing at the time.
When you arrive at Santa Monica, you will see God's Militia carrying out an execution on a young man for his crimes of eating seafood. This will end in bloodshed no matter what you do, you cannot convince God's Militia with words to stop the execution. If left to their own devices, the preacher will fire and kill civilians, so you'd better be swift with your weapons.
There is however a better way. Through speech, if you have high enough persuasion (I think it's Kiss Ass Lv 8, it's not very well documented) you can convince the preacher to let you shoot the boy yourself. This will stop the timer and give you ample time to plan your attack and place your squad members accordingly.