An Italian nun who was stabbed 19 times more than two decades ago by three teenage girls who were apparently fascinated by Satan has been honoured with beatification as a martyr of the Roman Catholic Church.

Hours before the ceremony in northern Italy, Pope Francis praised Sister Maria Laura Mainetti and described her killers as “prisoners of evil”.

Beatification is the last formal step in the Catholic Church before possible sainthood.

Italian news reports about the June 6 2000 killing in the town of Chiavenna said the defendants recounted during interrogation that the nun asked God to forgive her attackers even as they assaulted her.

After serving sentences ranging from eight and a half years to 12 years and four months, the convicted attackers were released from prison.

Two were aged 16 and the other was 17 when Sister Mainetti was killed, and they were prosecuted as minors.

Addressing the faithful in St Peter’s Square during his traditional Sunday remarks, Francis spoke of the beatification, saying Sister Mainetti, who belonged to the religious congregation of the Daughters of the Cross, was killed “by three girls influenced by a satanic sect. Cruelty”.

According to other Catholic clergy in the town, Sister Mainetti went out of her convent late at night after receiving a call from one of the girls who said she had become pregnant after being raped and needed help.

Prosecutors later described that as an invented story to lure the nun from the convent. She was struck repeatedly with a brick or a stone, then stabbed 19 times in an unlit alley.

Francis said Sister Mainetti “loved young people more than anything else, and she loved and forgave these same girls, prisoners of evil”.

The Pope said the nun’s legacy was her “life programme — do every little thing with faith, love and enthusiasm”.

The alley where the 60-year-old nun was killed is now a pilgrimage site. A bloodstained stone was taken to the Chiavenna sports stadium for the beatification ceremony.

Scribblings about the devil were reportedly found in the teenagers’ writings, prompting prosecutors to hypothesise that fascination with Satan was the apparent motive for the killing, but news reports later said one of the three reportedly told investigators they killed the nun to “do something different”.