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.
Antonio Salic told CNN he was working in a house across from the UPS building when he heard shots fired.
“I was working and suddenly I heard shots and I peeked out the window,” he said. “I saw people getting out (of the building), and they were wounded. A lot of people were running.”
The construction worker said he also saw people on the roof with their hands up, letting police know they were unarmed.
“Police approached and they were taken to a safe place,” he said.
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.