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San Francisco Examiner via AP, Mike Koozmin
Bystanders look on next to an open-air tour bus, at right under scaffolding, that crashed into a construction site near Union Square Friday, Nov. 13, 2015, in San Francisco. Officials say there may have been as many as 30 people aboard the bus. There's no word on the conditions of the injured.

SAN FRANCISCO — A double-decker tourist bus careened wildly out of control Friday in San Francisco's crowded Union Square, running down a bicyclist, at least two pedestrians and striking several cars before it plowed into scaffolding lining a construction site. Twenty people were hurt, including six critically.

Twelve people suffered minor injuries in the crash that happened just before 3 p.m., San Francisco Fire Chief Joanne Hayes-White said. The others suffered moderate injuries.

Union Square was crowded with shoppers and tourists when, according to eyewitnesses, the bus came roaring across two city blocks at a high rate of speed. 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 12-foot-high scaffolding.

It also knocked down several power lines used to propel the city's fleet of electrical buses.

Firefighters had 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 have been on board when the vehicle went out of control, officials said.

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.

Calls and messages for the bus operator, City Sightseeing San Francisco, weren't immediately returned.

Union Square is one of the city's most popular tourist destinations with several high-end stores, including Macy's and Saks Fifth Avenue, 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.

John Zimmer, who works at the park, said the bus didn't seem to slow until it hit the construction scaffolding.

He said he and others tried to put up a perimeter to keep tourists and others away from the live electrical lines until authorities arrived.

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 smashed up against the bus, along its sides or in its 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.

Virgil Canga was in a BMW with his two brothers when the bus rammed a car behind them and sent it into the back of the BMW.

"I heard a big bang, it was really crazy and I smelled burning," Canga told the San Francisco Chronicle (http://sfg.ly/1O81gts ) said. "I had to crawl out the door."

One of his brothers also climbed out but the third was flung around by the collision. He had to be rescued from the smoking wreckage and suffered chest and back injuries but was expected to recover, Canga said.

"I know it's Friday the 13th, but it's a lucky day for us," Canga said. "We survived. That's the way I look at it."

Eight patients, including a minor, were taken to San Francisco General Hospital. Five of the adults were in critical condition, spokesman Brent Andrew said.

Two people, one in critical condition, were sent to St. Francis Hospital, the Chronicle said.

Altogether, 16 people were hospitalized and four were treated at the scene, authorities said.

Associated Press writers Scott Smith in Fresno, California, and John Rogers in Los Angeles contributed to this story.