A view of St Peter's Square as Pope Francis celebrates Palm Sunday Mass at the Vatican, urging young people not to be silent and let their voices be heard: AP/Angelo Carconi

The Vatican has expressed "shame and sorrow" in its first response to a groundbreaking US Grand Jury report detailing decades of sexual abuse in the Catholic Church.

The report accuses over 300 "predator" priests throughout Pennsylvania of abusing nearly 1,000 children — and the Church of conducting a systematic cover-up. However, the actual number of total abuses in those dioceses since 1947 may be far higher than the reported figure. "We believe that the real number of children whose records were lost or who were afraid ever to come forward is in the thousands," the grand jury noted in its lengthy report.

In the Vatican's response, Pope Francis said he understands how "these crimes can shake the faith and spirit of believers," vowing to "root out this tragic horror."

"Regarding the report made public in Pennsylvania this week, there are two words that can express the feelings faced with these horrible crimes: shame and sorrow," said Greg Burke, director of the Vatican's Press Office. "The Holy See treats with great seriousness the work of the Investigating Grand Jury of Pennsylvania and the lengthy Interim Report it has produced. The Holy See condemns unequivocally the sexual abuse of minors."

The report includes harrowing details about the abuses priests allegedly carried out over the years. It also says the priests shared photos with each other of their sexual abuse victims.

"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," the report said.

Most of the accusations detailed in the new report are “too old to be prosecuted,” according to the grand jury, which found the majority had occurred before 2002.

That was the year US Catholic Bishops implemented new guidelines surrounding sexual abuse, which included removing accused clergy from office almost immediately and reporting allegations to the police.

"By finding almost no cases after 2002, the Grand Jury's conclusions are consistent with previous studies showing that Catholic Church reforms in the United States drastically reduced the incidence of clergy child abuse," the statement continued.

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Still, the Vatican said it "encourages continued reform and vigilance at all levels of the Catholic Church, to help ensure the protection of minors and vulnerable adults from harm."

"Victims should know that the Pope is on their side," the statement continued. "Those who have suffered are his priority, and the Church wants to listen to them to root out this tragic horror that destroys the lives of the innocent."