A Christian Democrat MP said Monday she is defending "freedom of speech and religion" on the first day of her hate speech trial in Finland over social media posts condemning homosexuality.

Paivi Rasanen, a former interior minister and Christian Democrats leader, denies all four charges of incitement against a minority group, which relate to a radio show appearance and online writings about same-sex relationships.

Rasanen described homosexuality as a "psychosexual developmental disorder" and said that homosexual people are "dysfunctional".

One charge related to a June 2019 tweet in which the long-time parliamentarian criticised the Finnish Lutheran church for partnering with that year's Pride celebrations, accusing the organisation of "elevating shame and sin to a subject of pride".

The post was accompanied by a photo of a New Testament passage which described homosexual acts as "shameful" and "unnatural".

"The statement is an affront to the equality and dignity of homosexuals and is likely to provoke contempt, intolerance and even hatred towards homosexuals," prosecution documents seen by AFP said.

Prosecutors called for the MP to receive a fine relative to her income which could exceed 13,000 euros.

A group of supporters had gathered outside Helsinki District Court on Monday morning, where Rasanen arrived carrying a bible.

She told waiting media she felt "honoured to be defending freedom of speech and religion."

"I hope that today it can become clear that I have no wish to offend any group of people, but this is a question of saving people for eternal life," Rasanen said.

Judges on Monday refused her legal team's application to drop one of the charges on the basis that it contravenes the right to freedom of religion set out in the European Convention of Human Rights.

Bishop Juhana Pohjola also faces incitement charges for publishing Rasanen's writings on the website of Finland's Luther Foundation.

When she was interior minister between 2011 and 2015, Rasanen voted Finland's current incitement laws through parliament.

"It never occurred to me that my own writings could one day be illegal," Rasanen said on Monday.