Internal documents from six Catholic dioceses in the US state of Pennsylvania showed that over 300 "predator priests" have been credibly accused of sexually abusing thousands of children, according to a grand jury report.
"We believe that the real number of children whose records were lost or who were afraid ever to come forward is in the thousands," CNN quoted the report as saying on Tuesday.
"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. Monsignors, auxiliary bishops, bishops, archbishops, cardinals have mostly been protected; many, including some named in this report, have been promoted."
The lengthy report investigates clergy sexual abuse dating back to 1947 in six dioceses: Allentown, Erie, Greensburg, Harrisburg, Pittsburgh and Scranton.
According to the report, the accused priests victimised boys, girls, teens and pre-pubescent children.
Some victims were plied with alcohol and groped or molested while others were raped.
"There have been other reports about child sex abuse within the Catholic Church. But never on this scale," the grand jurors wrote in Tuesday's report.
At a news conference announcing the report's release, Pennsylvania Attorney General Josh Shapiro called it the "largest, most comprehensive report into child sexual abuse within the Catholic Church ever produced in the US".
The report comes as the Catholic Church, including Pope Francis, is struggling to contain a sexual abuse scandal rapidly consuming the church on several continents.
In Australia, a bishop has been found guilty of covering up sexual abuse.
In Chile, the Pope was forced to recant his dismissal of an abuse scandal involving a prominent priest and bishops accused of covering up his crimes.