At least 20 people were hurt, six critically, when an open-air tour bus crashed Friday afternoon in San Francisco’s crowded Union Square.

The out-of-control bus ran down a bicyclist, two pedestrians and struck several cars before it plowed into scaffolding lining a construction site. San Francisco Fire Chief Joann Hayes-White said 12 people suffered minor injuries in the crash that happened just before 3 p.m.

Union square was crowded with shoppers and tourists when the bus quickly crossed two city blocks at a high rate of speed, according to eyewitnesses. It struck several moving vehicles in its path as well as the bicyclist and the two pedestrians, the latter ending up trapped underneath the vehicle after it plowed into the scaffolding.

Firefighters were forced to extricate the two people under the bus as well as one trapped on the top deck, Hayes-White said. As many as 30 people were believed to be on board when the vehicle went out of control. The San Francisco Chronicle reported firefighters had to remove seven people from the metal.

The bus knocked down several power lines used to propel the city’s electrical buses.

The driver was conscious and able to speak when firefighters pulled him from the wreckage, Hayes-White said. But she added it was too early to speculate about what caused him to lose control.

"The police department will investigate what those circumstances involved, whether it was mechanical failure, whether it was driver error. It's way too early to tell right now," she said.

Union Square is one of the city’s most popular tourist attractions with several high-end shops as well as its Union Square Park and skating rink. The scaffolding was in front of an upcoming Apple store.

The license for the company that operates the bus, City Sightseeing, was revoked in 2013, according to the Chronicle. However, officials for the California Public Utilities Commission told the newspaper the company had a permit to operate the bus.

Hoda Emam said she rounded a corner at the square immediately after the crash to see a chaotic scene unfolding, with emergency workers sprinting toward the injured as emergency vehicles arrived with their sirens blaring. Wrecked cars were scattered along the bus' path.

"Everybody was asking what's going on," said Emam, a Bay Area resident in San Francisco on personal business. "There were still ambulances and fire trucks and paramedics with stretchers coming up."

Police officers were on top of the double-decker bus apparently attending to the injured, she said, adding that another three or four people lay in the street being treated.

Six of the injured are in critical condition and being treated at San Francisco General Hospital, hospital spokesman Brent Andrew said. They are three men and three women between the ages of 20 and 60, he said.

Three of the injured were sent to the emergency room at St. Francis Hospital, hospital spokeswoman Robin O'Connor said. She declined to provide any further information, citing privacy issues.

The Associated Press contributed to this report.