SAN FRANCISCO » What began as a serene tour through one of the nation’s most picturesque cities turned into a two-block ride of terror when an open-air tour bus careened wildly out of control in San Francisco’s Union Square, running down a bicyclist, striking two pedestrians and smashing into several moving cars before it plowed into scaffolding lining a construction site.

Twenty people were hurt, six critically, when the big blue bus with at least 30 people aboard raced through one of the city’s most popular tourist destinations Friday afternoon, leaving chaos and carnage in its wake.

“Everybody was asking what’s going on,” said Bay Area resident Hoda Emam, who rounded a corner leading into Union Square moments after the bus struck the scaffolding, bringing it down in a twisted heap of metal and other debris.

She saw paramedics sprinting toward the injured while emergency vehicles pulled up with their sirens blaring.

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

“There were still ambulances and fire trucks and paramedics with stretchers coming up,” she said.

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.

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, Hayes-White said. But she added it was too early to speculate about what caused the crash.

“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 Friday.

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

The six critically injured were taken to San Francisco General Hospital, said hospital spokesman Brent Andrew. He identified them as three men and three women between the ages of 20 and 60.

Three other people were treated in the emergency room of St. Francis Hospital, said that hospital’s spokeswoman, Robin O’Connor. She declined to provide further information, citing privacy issues.

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 awhile to figure out who was who.”