San Francisco police are responding to a shooting at a building near the Bayshore Freeway, the department said in a tweet.
People in the area are advised to shelter in place until instructed otherwise.
Aerial footage showed police leading dozens of people, many in matching brown work uniforms, out of a building. A tactical vehicle arrived shortly thereafter.
The site of the shooting is the Potrero Hill area, a largely residential neighborhood.
Police seem to be concentrated around the UPS San Francisco Customer Center, a couple of blocks east of Franklin Square.
Residents have been warned to avoid the area because of heavy law enforcement activity. Authorities say they will provide a briefing soon.