A double-decker tour bus with an open-air top reportedly lost control and crashed into the construction site of the new Apple flagship store, at the corner of Post and Stockton, just before 3 p.m. today. NBC Bay Area had a chopper on the scene, showing scaffolding that collapsed on and around the bust, and ABC 7 reports that the injury total is now at 20, with five people in critical condition, including a bicyclist who was struck by the bus. The bus driver was also taken to the hospital.

From footage from the scene it appears multiple vehicles were involved in the crash with several wedged against the scaffolding by the bus, and a second open-air tour bus next to the one that crashed.

Witnesses have posted shots of the scene on Twitter, showing the injured being removed on gurneys. SF Weekly's Chris Roberts cites Union Square staffers who "say [the] double decker tour bus 'lost control,' ran into traffic."

Another report, via a witness who spoke to ABC 7, says that the bus driver was speeding and driving erratically for at least two blocks prior to the crash.

The investigation into the incident is, obviously, ongoing.

We'll update you as we learn more.

This post has been updated throughout.