The bomb squad is on the scene and police are working with hostage negotiators to get the person to surrender.
They received reports of a person armed with a gun shortly before 2 p.m. They say the suspect is barricaded inside a building with a gun, but there were no reports of shots fired.
Pedestrian and vehicle traffic on Valencia Street between 15th and 18th has been shut down and residents are asked to avoid the area.
A source says that this all started when a manager went door-to-door to tell tenants they were going to spray pesticides in the building. The source says that a tenant then suddenly drew a gun, and that started the police action.
Viewer Rebecca Pierce is inside Thanh Tam II Restaurant on Valencia Street and tweeted photos showing police outside.
Photos from inside Thanh Tam Il Restaurant on Valencia in SF where I'm currently under lockdown due to man with a gun down the street pic.twitter.com/jABCpvg3fY— Rebecca Pierce (@aptly_engineerd) March 10, 2017
She later tweeted video of police inside the restaurant and her leaving the restaurant and running down the street.
We were evacuated from Valencia, pretty orderly until we got outside and had to run. A few blocks away now and safe pic.twitter.com/Ia3DVk8biy— Rebecca Pierce (@aptly_engineerd) March 10, 2017
San Francisco police are posting updates on their Twitter page. They say their negotiation team is "creating time & distance in an attempt to apprehend the suspect w/ a peaceful outcome."
The closure of the busy thoroughfare is causing heavy traffic in the area. San Francisco Municipal railway buses are being rerouted.
Bay City News contributed to this report.