The accident happened at approximately 5:30 p.m. Saturday near the 31st Street exit off south-bound Lake Shore Drive.
"I heard people kind of start to freak out at the front of the bus, like, 'Oh God. Oh, God,'" said passenger Joe Hayes.
The number 6 Jackson Park express, packed with dozens of passengers, jumped a curb and kept on going.
"We plunged into that thicket on the side of the road. We might have hit 15 or 20 different trees until we hit a big one, a foot and a half around," said Andrew Wooldridge, also a passenger.
"There were children. There were handicapped. There were plenty of people on the bus," said passenger Rachel Henry.
Passengers say the impact sent people toppling throughout the bus. It stopped mere feet from a retaining wall drop-off.
"Had the trees not been there, had we not all been on that bus to distribute the weight, we would have flipped. We would have flipped over," Henry said.
"People were toppling over each other, and that's the way we kind of buffered each other," said passenger Modupe Sodamade.
The Chicago Fire Department rushed 21 ambulances to the scene.
Most passengers were able to walk away, except for the handful who were seriously and critically injured.
"Multiple contusions, possibly some fractures, normal injuries sustained in a motor vehicle accident. Back pain and neck pain," said Mark Levison, of the Chicago Fire Department EMS.
"About 10, 12 people could not move from where they'd been injured. So, they stayed on the bus," said Wooldridge.
"There's a couple people bleeding. I took my shirt off to put it on a guy's arm to keep from bleeding," said Hayes.
The bus driver was not seriously injured in the crash. She talked to CTA officials Saturday night.
"We'll be going ahead and doing our standard drug and alcohol testing on the driver, as well as taking a look at the mechanical system on the bus and looking at the black box and trying to get a better understanding on why the bus may have veered off the road," said CTA Pres. Richard Rodriguez.
All CTA buses are equipped with video cameras. ABC7 Chicago is told the tape from the bus involved in the accident will be studied by investigators. The CTA was not able to give ABC7 Chicago additional information about the bus driver, including her name of level of experience.
Officials said late Saturday all bus routes were running on schedule.