A landmark US investigation has found that more than 300 “predator priests” molested in excess of 1000 children in the state Pennsylvania in a case of institutionalised abuse dating back to the 1950s.

The grand jury report released Wednesday (AEST) echoes the findings of other church investigations around the country in its description of widespread sexual abuse by clergy and church officials’ concealment of the offending. 

The report, which covered six of the state’s eight Catholic dioceses, is the broadest examination yet by a government agency in the US of child sexual abuse in the Catholic Church.

Pennsylvania Attorney General Josh Shapiro said the probe found a systematic cover-up by senior church officials in Pennsylvania and at the Vatican.

The report catalogues horrific instances of abuse, including a priest who raped a young girl in the hospital after she had her tonsils out, and another priest who was allowed to retain his position after impregnating a 17-year-old girl, forging a signature on a marriage certificate and then divorcing the girl.

“Despite some institutional reform, individual leaders of the church have largely escaped public accountability,” Mr Shapiro told a media conference.

“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,” he said.

The grand jury report said it believed the “real number” of abused children might be “in the thousands” since some records were lost and victims were afraid to come forward.

Despite the damning findings, few of the priests identified are likely to face criminal charges.

The statute of limitations has run out on most of the abuse cases, the Associated Press reported.

In addition, more than 100 of the priests are dead and many others are retired, have been dismissed from the priesthood or put on leave.

Authorities evaluated each suspect and have so far been able to charge just two priests, including one who has since pleaded guilty.

Mr Shapiro said the investigation was ongoing.

Church officials “routinely and purposefully described the abuse as horseplay and wrestling” and simply “inappropriate conduct,” Mr Shapiro said.

“It was none of those things. It was child sexual abuse, including rape.”

The grand jury accused Cardinal Donald Wuerl, who leads the Washington archdiocese, of helping to protect abusive priests when he was Pittsburgh’s bishop.

Cardinal Wuerl, who led the Pittsburgh diocese from 1988 to 2006, disputed the allegations.

“While I understand this report may be critical of some of my actions, I believe the report confirms that I acted with diligence, with concern for the victims and to prevent future acts of abuse,” he said in a statement.

“I sincerely hope that a just assessment of my actions, past and present, and my continuing commitment to the protection of children will dispel any notions otherwise made by this report.”

-with agencies