A 68-year-old woman has been sentenced to 100 hours of community service after writing a three-page letter to an Islamic girls school claiming that Muslims worship Satan.

Great-grandmother Rose White was found guilty of sending an indecent or grossly offensive letter to Manchester Islamic High School for Girls.

White insisted that it was her "duty" to tell Muslims to "start worshipping Jesus" and has sent around 50 similar letters to other organisations.

In the note, the retired mill worker likened Mohammed to Satan, and said the school would "have a nice future under Jesus and not Satan or Mohammed or whatever they call him."

After seeing pictures of girls in burkhas on the school website, she added: "I was saddened to see you enforce full Muslim dress and force pupils to accept the role of Satan."

Accompanying her three-page letter was a 23-page cartoon booklet called Allah Like You?, which featured a cruel Muslim father whose conversion to Christianity transforms his character.

The headteacher of the school, Mona Mohammed, was said to be "deeply alarmed" by the note.

"She read the letter and was shocked to see the content was Islamophobic. It stated all Muslims were doomed to die and that Muslims worship Satan in the guise of Allah," prosecutor Subhanur Chowdhury said.

White denied having done anything wrong. "The letter is not offensive – it just told the truth. As a Christian it's my duty to tell them to start worshipping Jesus," she said.

Judge Duncan Birrell, however, ruled against her. "You are not allowed to conduct yourself in a way that causes other people harm," he said. "The court has already determined you have crossed the line between freedom to express your opinions and causing harm to others."

White was sentenced to a 12-month community order and a £510 bill.