Over 300 "predator" priests in Pennsylvania sexually abused thousands of children for decades and saw their crimes covered up by the Catholic Church, a new report found Tuesday, which details graphic cases of clergymen raping and abusing children, even toddlers.

The devastating 884-page report, based on half a million pages of church records, was made public by Pennsylvania Attorney General Josh Shapiro after a two-year investigation into six of the state’s eight dioceses, and described a "weaponization of faith" to silence victims and a systematic cover-up campaign by bishops.

It is thought to be the single most comprehensive report into abuse in the US church since The Boston Globe first exposed pedophile priests in Massachusetts in 2002.

Nevertheless, only two priests have been charged in connection with the crimes.

The majority of those responsible are dead and most of the crimes happened too long ago to prosecute, officials say.

Pope Francis celebrates an open-air mass at the Benjamin Franklin Parkway in Philadelphia, Pennsylvania, September 27, 2015. /VCG Photo

"Priests were raping little boys and girls, and the men of God who were responsible for them not only did nothing; they hid it all. For decades," Shapiro told a press conference Tuesday.

"The pattern was abuse, deny and cover up," he said. "As a direct consequence of the systematic cover-up by senior church officials almost every instance of child sexual abuse we found is too old to be prosecuted."

One priest forced a nine-year-old boy to give him oral sex, then rinsed out his mouth with holy water to "purify him," the report said.

Another abused five sisters from the same family, including one from the age of 18 months to 12 years. The diocese settled with the family after requiring a confidentiality agreement, Shapiro said.

More than 1,000 child victims were identifiable, but the "real number" was "in the thousands," the report estimated, taking into account lost records and victims who were afraid to come forward.

It called for changes in the law that would scrap the statute of limitations for child sex abuse, give victims more time to file civil lawsuits and tighten legislation compelling people to report abuse they find out about.

Shapiro said predator priests were allowed to remain in ministries for 10, 20 even 40 years after leaders learned of their crimes, or that bishops reassigned them repeatedly to different parishes.

Between 5,700 and 10,000 Catholic priests have been accused of sexual abuse in the United States, but only a few hundred have been tried, convicted, and sentenced for their crimes, according to the watchdog Bishop Accountability.