Francis demanded accountability in response to new revelations in the United States of decades of misconduct by the Catholic Church.
The Pope begged forgiveness for the pain suffered by victims and said lay Catholics must be involved in any effort to root out abuse and cover-ups.
He attacked the self-referential clerical culture that has been blamed for the abuse crisis, with church leaders more concerned for their reputation than the safety of children.
Francis wrote: "We showed no care for the little ones; we abandoned them."
The Vatican issued the letter on Monday, ahead of Francis' trip this weekend to Ireland which is expected to be dominated by the abuse crisis.
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