Forty five people have been arrested after police raided 50 homes in Greater Manchester in connection with alleged child pornography offences.
Operation Baglan is targeting child pornography on the web
Police swooped on the houses at 0700 BST and computers were seized.
Those arrested are due to be questioned later on suspicion of accessing indecent images of children on the internet.
Greater Manchester Police (GMP) said more than 500 officers were involved in the raids - codenamed Operation Baglan.
Det Supt Steven Heywood, who led the raids, said the operation was concerned with all levels of abusive images on websites, up to and including what is known as "Level Five" sites, which is the most serious rating ascribed to indecent images.
He added: "This kind of activity is not tolerated in Greater Manchester and we will continue to work towards its elimination."
He said GMP had the UK's "leading pro-active abusive images unit".
"The content of the images goes beyond pornography. Hundreds of children will have suffered to have fuelled these sites - accessing these sites is not a victimless crime," he added.
"This is a global problem, not just one that affects Manchester, but we will continue to do our best to protect the public."
Officers involved with Operation Baglan use electronic "footprints" left by computer users when they visit a website to gather intelligence.
GMP said that due to the scale and the sensitivity of the operation, full details of the homes raided would not be released at this stage.
However, they confirmed there were eight arrests in Manchester, three in Salford, four in Tameside, five in Stockport, six in Bolton, seven in Wigan, four in Trafford, one in Bury, four in Rochdale and three in Oldham.