By Nadine Schmidt, Jessica Benne and Lauren Said-Moorhouse, CNN

(CNN) -- A new report from Germany's Catholic Church admits to "at least" 3,677 cases of child sex abuse by the clergy between 1946 and 2014, according to local publications Die Zeit and Spiegel Online who have seen a leaked copy.

The report -- commissioned by the German Bishops' Conference and conducted by the Universities of Giessen, Heidelberg and Mannheim -- has taken four years to assemble and will be released in full on September 25.

According to both media outlets, the study says the victims were mostly boys, more than half of whom were aged 13 or younger. Every sixth case involved a rape and at least 1,670 clergy were involved, the report also reveals.

"We know the extent of the sexual abuse that has been demonstrated by the study. We are dismayed and ashamed by it," Bishop Stephan Ackermann said in a written statement to CNN responding to reports of the leak.

Ackermann said in the statement that the aim of the study was to "gain more clarity and transparency about this dark side of our church, for the sake of those affected, but also to see for ourselves the wrongdoings and to ensure this cannot be repeated."

The statement continued: ''It is about a responsible and professional investigation. I am convinced that the study is a comprehensive and thorough survey that provides figures and analysis from which we will continue to learn. This also applies to the findings, which provide a deeper insight into the perpetrators' actions and the behavior of church leaders in recent decades. Again, I emphasize: The study is a measure that we owe not only to the Church, but above all, and first and foremost, to those affected.''

The Bishop also said it was regrettable that the report was made public "as not even the members of the German Bishops' Conference know the full study."

The German Bishops' Conference will initiate a hotline for the public to call a day before the scheduled publication later this month.

Separately on Wednesday, Pope Francis called for an extraordinary meeting of top Catholic officials to discuss the escalating abuse scandal that has made headlines in recent weeks.

Francis will meet the presidents of the Catholic bishops conferences from around the world in the Vatican from February 21-24, according to a Vatican statement on Wednesday.

The pontiff has faced mounting criticism over his handling of historic clerical sex abuse allegations. Last month, a former Vatican ambassador, Archbishop Carlo Maria Vigano, said he had informed the Pope about claims regarding former top American cardinal Theodore McCarrick in 2013 but no further action was taken.

"Pope Francis must be the first to set a good example to cardinals and bishops who covered up McCarrick's abuses and resign along with all of them," Vigano said in a lengthy statement dated August 22.

Francis is scheduled to meet Cardinal Daniel DiNardo, president of the US Conference of Catholic Bishops, on Thursday where they are expected to discuss the allegations surrounding McCarrick.

Correction: This story has been updated to correct the number of cases to 3,677.