20 injured in tour bus crash at San Francisco's Union Square

SAN FRANCISCO (KGO) -- Twenty people were injured, including six critically, after a tour bus collided with several vehicles at a construction site in downtown San Francisco. The crash happened Friday afternoon at Post and Stockton streets where a new Apple store is being built.

Officials say all six of those with critical injuries are being treated at San Francisco General Hospital. They include four men and two women between 20 and 60 years of age. The other 14 victims were taken to other hospitals.

According to officials, the driver of that open-air double-decker bus was driving erratically and speeding for at least two blocks before ending at the site.

Police say he hit a bicyclist, who is in critical condition, before hitting several vehicles and scaffolding which then fell on more cars and people. That scaffolding is part of construction at the site of the new Apple store.

PHOTOS: Several injuries reported after tour bus accident in SF

"According to eyewitnesses, they saw the bus, which is double-decker open-air bus, driving erratically down Post Street," said San Francisco Fire Department spokesperson Mindy Talmadge. "In the process, a few vehicles were struck then the bus came to rest against some scaffolding which fell into the street."

First responders had to pull people out because they were pinned beneath the bus and scaffolding.

"It was like a bomb going off. It just kept going boom, boom, boom, boom, boom like it wasn't gonna stop," witness John Zimmer said.

Police say they will investigate to determine what caused this accident.

"We will look at everything," said Officer Albie Esparza. "We will look at any mechanical malfunctions, we will look at any DUI or alcohol or narcotics as standard operating procedures go for collisions."

Officials say some victims were treated at the scene but others, including the driver and the bicyclist, were taken to the hospital.

The hospital has said very little about the nature of their injuries, but a source inside San Francisco general filled us in on some of the details. Of the five critical patients, two have major head injuries and one has an extremely serious broken leg. Those three patients are scheduled to be in surgery on Saturday.

Friends and family members of some of the victims were at the hospital Friday evening.

There were earlier reports that PG&E lines had been affected, but that is not the case. The lines that fell belong to Muni, so crews were busy Friday evening repairing them.

It took hours to remove the scaffolding and tow away the damaged vehicles, so streets in the area were closed until late Friday night.