The Internet Watch Foundation said it had made the first attempt to find out how many sites peddle abusive images and videos of children.
Its researchers found about 3,000 sites, with more than three-quarters run as commercial operations, typically by criminal gangs trying to make money out of the images.
"This is the first time any organization has revealed the true scale of this issue and been clear that the problem is something that can be solved," the watchdog said in a statement.
Chief Executive Peter Robbins said the new figure would help build the case for a global drive to eradicate the sites.
He said: "A coordinated global attack on these Web sites could get these horrific images removed from the Web.
"Speculative figures can create a distorted picture of the scale of the problem of child sexual abuse Web sites," he added.
The number of child abuse sites has remained static over the last few years, despite the growth of the Internet, he added.
The watchdog's annual report called for a worldwide campaign by governments, police, and the Internet industry to investigate and disrupt abusive sites.
Computer networks in Russia and the United States host the most child abuse images, although many other countries are involved, a watchdog spokeswoman said.
It can be hard to shut illegal sites because operators constantly switch countries, temporarily close them, or hop between different Internet hosting companies.
The victims come from many countries, although it is hard to pinpoint exact locations, the spokeswoman added.
"Child identification is an extremely difficult process," she said. "We often find that new material will surface in a noncommercial area...and those same images will appear on the commercial Web sites a year or so later."
Since 2003, less than 1 percent of child abuse content has been hosted on U.K. computers, down from 18 percent in 1997, the report says. Sites hosted in Britain are closed within hours.
During 2007, the majority (71 percent) of global sites were "live" for less than 50 days of the year, the report said.
It also highlighted a significant problem with pedophiles sharing images between themselves online.
Set up in 1996, the Internet Watch Foundation is a self-regulating charity funded by the European Union and the Internet industry. Its role is to remove child abuse, criminally obscene material, and racist content from the Internet.