At least 20 people were injured when a double-decker tour bus crashed into several cars and careened into a scaffolding in downtown San Francisco on Friday afternoon, authorities said.

Six people were in critical condition, 12 were transported to area hospitals with various injuries and two were treated on the scene, said San Francisco Police Sgt. Michael Andraychak.

San Francisco Fire Department spokeswoman Mindy Talmadge said the incident started around 3 p.m. local time. Witnesses reported the bus struck a bicyclist before crashing into about five other cars and the scaffolding, which then collapsed onto vehicles and people below, near the city's famed Union Square, Talmadge said.

She said one rider who was on top of the open air level of the double-decker bus was critically injured and had to be removed by firefighters. The driver was also hurt, and several people were found either underneath the bus or had to be pulled out of their cars, Talmadge said.

Talmadge said the cause of the crash was not immediately known.-Reuters