A brutal assault on a downtown San Francisco sidewalk captured on a seven-second video clip and publicized by a startup CEO whose employee witnessed the incident has led police to seek the public’s help in identifying the suspect.

In the clip, which was recorded around 1 p.m. Tuesday, a man in a gray sweatshirt and pants and a New York Yankees ballcap is seen savagely stomping on the neck of another man who was lying on the sidewalk.

The assault took place on Mason Street just north of Market Street, San Francisco police said.

In the brief video, an unseen man is heard to holler “hey!” at the man in the sweatshirt, who raises a hand and replies, “Don’t talk to me, sir.”

The victim was identified by police only as a 38-year-old man. The suspect, who initially punched and knocked the victim to the ground, fled on foot, northbound on Mason Street, police said.

Circumstances of the assault, and whether the victim and suspect knew each other, are being investigated, said Officer Adam Lobsinger, a police spokesman.

The assault took place in front of the office of Hvmn (pronounced “Human”), a nutritional supplement startup. CEO Geoffrey Woo said one of his employees saw the altercation begin as a verbal argument, then began filming after it turned violent. Woo posted the video clip on Twitter. It has been viewed 25,000 times.

“General lawlessness happens on our block every day,” he said. “Open air drug sales. Car windows smashed. Fights. It seems to be a no man’s land. When the police are there, things are better. But they’re not always there.”

Anyone with information can call the police tip line at 415-575-4444 or text TIP411.

Correction: A previous version of this story misquoted the man seen in the video and misidentified the man who tweeted about the incident.

Steve Rubenstein is a San Francisco Chronicle staff writer. Email: srubenstein@sfchronicle.com. Twitter:@SteveRubeSF