Rete l'Abuso did not say how far back the alleged abuse cases occurred.
The Italian Bishops Conference (CEI), which was criticised by the Vatican's child protection commission last week, had no comment on the findings, a spokesperson said.
The global Catholic Church has been shaken for decades by scandals involving pedophile priests and the cover-up of their crimes, but local church leaders in Italy have been less forthcoming in confronting the issue.
New Pope Leo, who met survivors of sexual abuse by clergy for the first time this week, has told the Church's newest bishops not to hide misconduct allegations. His predecessor, the late Pope Francis, made addressing the issue a priority of his 12-year papacy, but with mixed results.
In an unusually critical report on the issue published on October 16, the Vatican's child protection commission said only 81 of Italy's 226 dioceses responded to a questionnaire about safeguarding practices it had drafted.
Rete l'Abuso said it had documented 1250 suspected cases of abuse - some with multiple victims - including 1106 allegedly committed by priests, with the rest attributed to nuns, religion teachers, lay volunteers, educators and scout members.
Its report contained cases related to 4625 victims - or survivors, as the association calls them - including 4395 abused by priests.
The report said 4451 of the survivors were under 18, and an almost equally large number - 4108 - were male, Rete l'Abuso said, adding that five nuns, 156 vulnerable adults and 11 people with disabilities were also among the victims.
According to the association, out of the 1106 suspected predator priests, only 76 were subjected to church trials, with 17 temporarily suspended, seven moved to other parishes and 18 were defrocked or resigned from the priesthood. Five died by suicide, it added.
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