A Finnish Christian democrat MP and former minister of the interior went on trial on Monday for agitation against gay people over comments in which she described homosexuality as a developmental disorder and a sin.

In the unprecedented case over freedom of speech, the Helsinki district court must also decide whether citing the Bible can be considered a crime in some cases in Finland.

Paivi Rasanen, a medical doctor and MP for a small Christian-democratic party since 1995, called homosexuality "a developmental disorder" in an online opinion letter published in 2004 and "a shame and a sin" on Twitter in 2019, the prosecutor said.

In a programme broadcast by Finnish public radio station Yle in 2019, Rasanen described homosexuality as a form of "genetic degeneration", the prosecutor added, charging her with three counts of agitation against gay people.

Rasanen has denied any wrongdoing and said the prosecutor's claims were unfounded.

"I feel great responsibility because I am aware that this case is historic for freedom of speech and freedom of religion," she told Reuters in an email ahead of the trial.

"Obviously, Christians sticking to the Bible's teachings have the right to participate in public debate," she said.

In her 2004 opinion piece, Rasanen claimed that scientific evidence proved undeniably that homosexuality was a disorder in a person's psychosexual development and she suggested that gay relationships early in life could lead to sexual abuse by adult men.

In her 2019 comment on Twitter, Rasanen questioned the Finnish Evangelical Lutheran state church's decision to partner with a leading LGBTI rights association to organise a Pride event, and posted a picture of a quote from the Bible condemning gay relations.

The prosecutor says Rasanen should be fined and the public broadcaster ordered to remove parts of the radio programme. 

The court will rule on the matter on a later date.