Pope Francis has condemned the “atrocities” of child sex abuse and cover-ups by the clergy in an open letter to the world’s 1.2 billion Roman Catholics.

Addressing a US Grand Jury report revealing decades of alleged sexual abuse inside the Catholic Church in Pennsylvania, the Pontiff described allegations against the Church as “crimes that inflict deep wounds of pain and powerlessness” in victims, their families and the religious community.

“The heart-wrenching pain of these victims, which cries out to heaven, was long ignored, kept quiet or silenced,” he wrote. “But their outcry was more powerful than all the measures meant to silence it, or sought even to resolve it by decisions that increased its gravity by falling into complicity”.

The report detailed at least 1,000 cases of alleged sexual abuse committed by 300 “predator” priests in Pennsylvania — and what was a systematic cover-up by Church officials.

The majority of cases included in the report occurred before 2002, when the US Catholic Bishops adopted strict guidelines towards sexual abuse reports, including immediately contacting local police and immediately removing accused clergy.

In its report, the US Grand Jury wrote that the actual number of sexual abuse victims across several dioceses in Pennsylvania was probably “in the thousands” thanks to underreporting of potential crimes.

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“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.

Pope Francis decried the alleged abuses in the open letter published on Monday, writing: “It is essential that we, as a Church, be able to acknowledge and condemn, with sorrow and shame, the atrocities perpetrated by consecrated persons, clerics, and all those entrusted with the mission of watching over and caring for those most vulnerable.”

“We showed no care for the little ones; we abandoned them,” he continued. “The extent and the gravity of all that has happened requires coming to grips with this reality in a comprehensive and communal way. While it is important and necessary on every journey of conversion to acknowledge the truth of what has happened, in itself this is not enough.”

Vatican was warned about paedophile priests in 1963 Show all 2 1 /2 Vatican was warned about paedophile priests in 1963 Vatican was warned about paedophile priests in 1963 Pope Benedict celebrates Mass in Rome yesterday. The Vatican was beset by fresh revelations that it was warned of sex abuse by US priests 50 years ago REUTERS Vatican was warned about paedophile priests in 1963 Pope Benedict XVI washes feet during Mass at St John Lateran Basilica in Rome before celebrating the Chrismal Mass at St Peter's Basilica GETTY IMAGES/ REUTERS/ AP

Despite vowing reform and paying billions of dollars to victims of abuse, the Church continues to suffer from allegations of sexual misconduct.