A man on the scene has informed us that a suicidal person is threatening to leap off a building on Valencia between 16th and 17th Streets, and the area has been closed off since around 5:45.
Police spokeswoman Sergeant Lyn Tomioka confirmed that the 500 block of Valencia -- the "old Valencia Gardens" is off-limits to the public because of the potential jumper. Details were sparse -- including, even the sex of the person atop the building. Negotiator teams are purportedly in place.
More as we know more.
UPDATE: 10:30 P.M.: The SFPD informs us that negotiations are ongoing.
UPDATE: THURSDAY, 8:20 A.M. Police at Mission Station confirmed that the would-be jumper, a white male, was talked down by the SFPD and firefighters shortly after midnight. He is currently at General Hospital's psychiatric wing. The man's identity and rationale for contemplating suicide haven't yet been released to the public.