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Possible Answer 1:
I would immediately raise an alarm by dialing 911; then I would try to diffuse the situation by trying to talk to the perpetrator calmly and by keeping him engaged, while maintaining eye contact.
It can be difficult for a person to do something illegal or unlawful while looking another person in the eye, and that is why many criminologists and hostage-negotiators recommend not losing contact with the perpetrator.
If the potential attacker has a weapon like a knife or a gun, more decisive action is called for, where I would have to step in and disarm him.
Possible Answer 2:
If the potential attacker seems agitated at all or frightened, then it might be difficult to reach him or reason with him.
In most cases, a truly desperate man, who thinks he has nothing to lose, might react very badly to perceived threats, thereby exacerbating the situation.
If it looks like he might soon lose control, then I would try to preempt the attack by disarming him, after calling for help.