A man has been freed from beneath a bus that hit him outside the Adelaide Festival Centre in the city's CBD.

Police said the accident happened when he ran onto King William Street about 11:20am.

Emergency services were called and worked to free him from beneath the vehicle.

The 44-year-old man was conscious when removed from under the bus.

He is reported to be in a stable condition at the Royal Adelaide Hospital.

The bus driver has been interviewed by police.

Colleague Steven Higgins was on another bus on King William Street when he heard an alert on his radio.

"I was parked down there but I just heard him [the driver] on the radio and then come up and the guy was under there so still pretty shaken up," Mr Higgins said.

"It's going to be a long time before he [the driver] recovers that's for sure.

"It's not something that happens all the time. I'm pretty shaken up just by seeing it."

King William Road was closed for several hours while police investigated, but the road is now open.