Within an hour, and with incredible Chinese-whispers efficiency, the Twitterverse became a hurricane of rumour, retweets and rampant speculation as the online massive tried to get to the bottom of an (admittedly very frightening) rumour.
Tweets warning people to stay inside, or to stay away from Oxford Street flooded people's feeds, and the term 'Oxford Street' quickly became the top trending topic on the Twitter homepage.
Quickly though, reports that the police had heard of no such thing started to circulate, and after about an hour the insanity had died down, but not before more than 1,000 tweets had been posted on the subject.
So what started the panic? We can't be sure, but the most plausible theory at the time of writing is that it all began when Nicola Whitehead tweeted, "There's a GUNMAN on Oxford Street -- please keep EVERYONE inside -- NO JOKE. Armed Police are on the scene."
She claimed she'd received an email announcement from film and music network UK Screen, who apparently had received a warning from the police. She quickly apologised, saying it was a misunderstanding, and that she "really didn't mean to cause any harm -- I fear it's gone a bit 'viral' though..."
"I was RELIABLY misinformed by an Official source and was GENUINELY concerned for the safety of my fellow Londoners," she added.
In an illustrating little parable, fashion assistant Candice Bailey was initially blamed for tweeting the following: "Street style shooting in Oxford Circus for ASOS and Diet Coke. Let me know if you're around!!"
That tweet was about a photography shoot, and was misinterpreted, but seems to have merely been a goofy coincidence -- and a better story for people to pass on. Thinking she had caused a panic, she apologised too.
Amid the chaos, tweets appeared such as "is there a shooting in Oxford Circus right now???? this is what i'm hearing" and, "Apparently theres a shooting at Oxford Circus, gunmen on the move. Guess I'm not gonna have McDonalds lunch then."
As the panic calmed, we spotted tweets, firing off like rounds from some kind of weird social-media machine gun, advising that Godzilla was responsible, as well as the odd rickroll.
Sky News' Tom Rayner confirmed Whitehead's story, tweeting that the Met Police had told him that "information about a routine police training exercise inadvertently got into the public domain", so one of the initial shooting tweets could have been about that, perhaps chiming with Bailey's tweet. Trainees "were given hypothetical details of an armed incident", according to Rayner.
With claims on Twitter popping up faster than they can be verified, this kind of thing is bound to happen occasionally. Users of the site are typically just as quick to scotch a rumour as to pass one on.
Happily this one turned out to be false, but at least if something monumental were to happen, those on Twitter would hear about it first, and perhaps could get to some kind of massive bunker. With Wi-Fi obviously.
Note: We used Twitter's search tool to estimate the number of Oxford Street-related tweets.