How a disturbing Facebook post may have prevented a mass shooting at the Women's March.

On the morning of the third annual Women’s March, a horrific tragedy was potentially prevented by swift-acting police.

It was Saturday, 19 January and 27-year-old Utah man Christopher Cleary had allegedly threatened to carry out a mass-shooting and kill “as many girls as I see”.

How do we know this? Because police say the Provo man wrote about his alleged intentions in a Facebook post – the same Facebook post that led to his arrest at a McDonald’s restaurant early on Saturday.

Police arrested the man after seeing what they say was his disturbing message, HuffPost reports.

"All I wanted was a girlfriend," Cleary allegedly wrote.

"All I wanted was to be loved, yet no one cares about me I’m 27 years old and I’ve never had a girlfriend before and I’m still a virgin.

"This is why I’m planning on shooting up a public place soon and being the next mass shooter cause I’m ready to die and all the girls the turned me down is going to make it right by killing as many girls as I see."

Police tracked the man, who was on probation Colorado for stalking and threatening women, to the fast-food restaurant. It was located just a few hundred metres away from the site where women were planning to unite for Provo’s Women’s Wave Rally.

There, he was arrested for the felony of threatening terrorism.

In the subsequent police interview, Cleary told police he made the threats "but claimed to be upset and not thinking clearly when he did so", the Washington Post reports.

A court date for his first appearance has not yet been set.