A window washer fell screaming about 11 stories from the top of the Sterling Bank and Trust building in San Francisco Friday, landing on a car in the middle of a busy street, police and witnesses said.

The man suffered critical injuries, but he was conscious, and the driver was not injured, police said.

San Francisco Police Lt. Ed Del Carlo said the worker was getting ready to work when he "fell off the apparatus" and landed on a car about 10 a.m. after falling from the building at 400 Montgomery Street, near the intersection with California Street. The roof of the car, a green Toyota Camry, was smashed in, and the rear windshield shattered.

"The driver didn't know what happened," Del Carlo said.

The driver, Mohammad Alcozai, at first thought it was a bicyclist who accidently hit his car. But when he got out of his car, Alcozai said he couldn't believe what he saw.

Window washer falls 11 floors and hits this Toyota at California and Montgomery, San Francisco pic.twitter.com/mvv4aUTIif — Mark Matthews (@MarkMatthewsNBC) November 21, 2014

"Somebody said they fell off the roof and I look and said, 'Oh my God,'" he said. "And when I see my car it was flat -- only my spot was OK."

Alcozai quickly ran to the man's aid.

"He was shaking pretty bad. He was shaking and wasn't able to talk," Alcozai said. "He was breathing hard."

The window washer, who has not been identified by police, was taken to San Francisco General Hospital where he remained in critical condition late Friday afternoon. Cal/OSHA spokeswoman Julia Bernstein said the man suffered a broken arm and injuries to his side. He was with a partner, who was not hurt. Bernstein said a safety engineer was on scene, trying to determine what happened.

The intersection of Montgomery and California was closed to traffic for several hours as police investigated.

Sam Hartwell, who was on his way to a meeting, saw some of what happened: "I saw a blue streak out of the corner of my eye," and then that "streak" hit a car with a great "thud." Soon afterward, Hartwell realized that "streak" was a person.

Hartwell and about 20 other people ran to the man, who was on his back. The man was lucid, though he was bleeding.

"He understood we were with him,'' Hartwell said.

The fire department says a paramedic was at the scene within five minutes of the first 911 call, but it took 10 minutes from the time of the call for an ambulance to arrive.

The bystanders, who included a nurse, put clothing on the man as they waited for the ambulance.

Hartwell said of his reaction, "It was utter, immediate shock. How do you react to something like that?''

The window washer worked for Century Window Cleaners of Concord, which has been fined for safety violations in the past. A complaint from 2008 resulted in a $2,700 settlement. The state ordered the company to train and “supervise the use of equipment and safety devices to insure that safe working practices are observed.”

NBC Bay Area contacted the company for comment, but the man who answered the phone declined. The company's website states it carries a $5 million worker compensation insurance policy and a $5 million general liability insurance policy.

The fall comes about two weeks after two window washers were stranded on top of the World Trade Center in New York City. On Nov. 12, two workers were rescued in dramatic fashion after scaffolding collapsed.

Last month in Irvine, California, two window washers stuck for hours near the top of a 19-story high-rise were pulled to safety by members of a search-and-rescue team.

Window cleaning is one of the safer industries, according to Stefan Bright, the safety director for the International Window Cleaners Association based in Zanesville, Ohio.

Among the 15,000 to 20,000 professional cleaners working on high-rises each year, there are typically fewer than three fatalities a year, he said.

While figures for window washers specifically were not available, the Bureau of Labor Statistics reports that seven workers in the janitorial or cleaning professions died as a result of on-the-job injuries sustained while working with scaffolding from 2011 to 2013.

The Associated Press and NBC Universal's Noreen O'Donnell and Torey Van Oot contributed to this report.