More than seven websites showing child abuse are reported to the police every day, an online charity has said.
Nearly one in three children depicted appear to be under six
The Internet Watch Foundation (IWF) also said new research showed the images, which included rape, sadism and bestiality, had become more severe.
Now web giants including Tiscali, Yahoo and MSN are helping to publicise the IWF's hotline, which allows internet users to report these sites.
About 1.5m UK adults had seen child abuse online, the IWF said.
It also said more than a third of all child sexual abuse sites contained images of the most severe kinds of abuse.
Nearly one in three children appearing on the sites were under six years old, while one in 20 were under the age of two, the IWF said.
Vigilance in UK
The charity's chief executive, Peter Robbins, said: "Our analysts witness the results of terrible sexual abuse being inflicted on very young children around the world and then circulated online.
"With the help of the online industry, the 28 hotlines we work with around the world and our law enforcement colleagues, the public can help us to remove these websites and end the abuse that is perpetuated every time the images are viewed."
Through the hotline, the IWF offers a "notice and take-down" service to police and internet service providers.
The internet companies will run adverts on their websites and among e-mail customers as part of an awareness day.
The charity said of all the sites reported to them in 2007 just 1% could be traced back to a UK server, compared with 18% 10 years ago.
The IWF said so far this year it had passed on details of 2,092 child sexual abuse websites to police and child protection agencies.
The awareness day will be followed by a series of regional presentations for professionals concerned with tackling online sexual abuse of children.