SAN FRANCISCO » Four adults remain in critical condition at a San Francisco hospital after an open-air tour bus careened out of control Friday, running down a bicyclist, striking two pedestrians and smashing into several moving cars before it plowed into scaffolding lining a construction site.

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 Friday night.

What began as a serene tour through one of the nation’s most picturesque cities turned into a two-block ride of terror as the big blue double-decker bus careened through San Francisco’s Union Square before it plowed to a stop amid a heap of twisted metal.

Twenty people were hurt and at least 30 passengers were onboard when the crash occurred shortly before 3 p.m.

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.

Officials are still trying to determine what happened, San Francisco Police Department spokesman Albie Esparza said Saturday.

City Sightseeing San Francisco chief executive Christian Watts issued a statement by email Saturday, saying that the company is "deeply saddened" by the crash.

"We are cooperating with the authorities to determine how this happened. We pray that everyone involved recovers fully and quickly," he said.

At St. Francis Hospital, hospital spokeswoman Robin O’Connor said that of three people admitted to the hospital Friday, one man remained in stable condition Saturday morning.

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.

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

Witnesses said it 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.

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

John Zimmer, who works at Union Square Park, said the vehicle never appeared to slow down until it struck the scaffolding. He added that he and others tried to put up a perimeter to keep tourists and others away from the live electrical lines until authorities arrived.

Union Square was so crowded that he said it was initially difficult to separate some of the injured from the passers-by.

"I couldn’t tell who was a tourist and who was an accident person," he said. "It took us a while to figure out who was who."

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