It was a scene of apparent mayhem in SoMa Saturday afternoon, complete with screaming citizens, a squirming man on the ground, and at least one cop running around with a shotgun. And according to police, it all began with a threat made on the bus.

SFist reader Ryan Blake was riding the 14-Mission Muni bus Saturday at around 3 p.m. "on my way to Hard French with friends." That's when, he says, all the riders were told to disembark.

"When we started walking we ran into this scene," at 11th and Mission Streets, he says, sending along this video:

According to my esteemed colleague Jay Barmann, who was passing through the area by car at around the same time, "the cops had shut down the entire block" during their response to the situation.

Congestion in the area appears to have been exacerbated by a Muni collision at around 1:54 p.m. that knocked down the overhead lines and snarled Muni travel.

But, obviously, it takes more than fallen Muni wires to elicit that kind of police response! So, what gives? According to an email from San Francisco Police Department spokesperson Officer Carlos Manfredi, "The incident was regarding a dispute that occurred on the Muni bus," presumably the one you see parked to the right.

"The subject with the back pack threatened to cut another man who was with his children," Manfredi writes, and "was seen with the knife by an officer on scene."

Update: As commenters have noted, the shotgun seen in the video isn't one that shoots bullets. Via Twitter direct message, Manfredi says that "The shotgun seen is a less lethal bean bag...The less lethal is a safe method to stop a threat. Especially one carrying knives. Orange stock means bean bag shotgun."

According to Manfredi, the alleged bus threatener "was later arrested for terrorist threats and child endangerment."

And then, by 4:45, Muni travel resumed in the area, and peace returned to SoMa.