SAN FRANCISCO -- Hundreds of thousands of disturbing videos of young children have been posted online, not in some dark corner of the internet, but on YouTube.

The website is finally cracking down. It was a bottom-line decision.

Under pressure from advertisers, YouTube -- owned by Google -- removed more than 150,000 videos with images of children apparently being assaulted.

"At the end of the day, large tech companies are run by the bottom line, and I think YouTube has been an incredibly successful platform for Google, and they have simply not paid enough attention to some of the bad stuff that happens on that platform," said James Steyer, CEO of Common Sense Media.

YouTube began paying attention as advertisers pulled their ads, complaining about child-endangering videos.

In a statement Tuesday, YouTube said:

"Over the last few months we've taken deliberate steps to tackle many of the emerging challenges around family content on YouTube. In the past week, we've taken aggressive action, including removing over 150,000 videos from our platform that do not belong on YouTube, turning off inappropriate comments from over 625,000 videos, terminating hundreds of accounts and adding age-gates for videos that are clearly geared towards mature audiences. Beyond that, we have removed ads from over 2 M videos and over 50k channels that masquerade as family-friendly to reduce the incentive to produce this content."

Mary Pulido CBS News

"It's a new type of child abuse -- child sexual abuse exploitation," said Mary Pulido, executive director of the New York Society for the Prevention of Cruelty to Children. "Children can't unsee something. Once they look at an image, it is there in their brains forever."

The problem on YouTube is that with some 300 hours of video uploaded to the site every minute, identifying offensive material is a constant challenge. YouTube has largely depended on computers to determine what is appropriate.

"A machine, a computer, cannot do that as well as a human being," Steyer said.

"So do you have to hire hundreds and hundreds and hundreds of people to look at everything?" CBS News asked.

"If you have to hire hundreds and hundreds and hundreds of people to look at everything, they should, because Google is coining money," Steyer said.

While YouTube is taking steps to protect children, experts say the first line of defense is at home -- that parents have to be aware of what their children are watching and should limit the amount of time children spend watching online videos.