The harrowing CCTV video shows the moment the bus ran down a cyclist and struck two pedestrians.

The runaway bus then smashed into several moving cars before ramming into scaffolding at a construction site, continuing downhill toward Union Square located downtown.

Twenty people were critically injured, but miraculously no one died.

San Francisco General Hospital spokesman Brent Andrew said six victims still in its care suffer from facial and head injuries as well as fractured ribs, arms and legs. Some have organ injuries.

The patients in critical condition are two men and two women. Two other men are in serious condition. Andrew declined to share any details of a minor who was also admitted to the hospital on Friday night.

Firefighters had to extricate the two pedestrians, who ended up trapped under the bus, and a passenger trapped on the upper deck.

The driver was conscious and able to speak when firefighters pulled him from the wreckage, Fire Chief Joanne Hayes-White said.

But she added it was too early to speculate about what caused the crash.

Union Square is one of the city's most popular tourist destinations with several high-end stores as well as its Union Square Park and skating rink.

The scaffolding was in front of what is going to be a new Apple store.

Several upscale hotels are also located near the square, which was crowded with shoppers and tourists on what had been a pleasant day.

Witnesses said the bus raced across two city blocks, hitting the bicyclist, the pedestrians and striking several moving cars before ramming into the scaffolding.

Wrecked cars were scattered up and down the street.

San Francisco lawyer Robert Cartwright has been hired to represent bus driver Kenneth Malvar, 52, who was behind the wheel of a tour bus that crashed Friday afternoon in San Francisco's popular Union Square shopping district.

Cartwright said Malvar gave a brief account from his hospital bed.

Malvar said the brake pedals didn't work and the accelerator pedal felt "floppy."

San Francisco Police Sgt. Michael Andraychak said the accident is still under investigation.