THE Australian Sex Party is planning a demonstration in response to a "prayer offensive" launched by a religious group to counter what it says are the dark forces at work in Canberra.

Sex Party convenor Fiona Patten said she and others would conduct a peaceful demonstration on Canberra's Mount Ainslie on Saturday afternoon to protest the influence of radical religious groups on federal politics.

That will coincide with a gathering organised by Catch the Fire Ministries Pastor Daniel Nalliah to combat evil forces including witchcraft, homosexuality and abortion.

This was prompted, he said, by the discovery of a black mass altar at Mount Ainslie by a group of school students.

"The type of altar discovered on Mount Ainslie pointed to a black mass and the work of dark forces wanting to cast spells on Australia and federal parliament," he said in a statement on Monday.

Ms Patton suggested the chairman of the Senate privileges committee, Senator George Brandis, could attend to ensure no MP was threatened by Pastor Nalliah's exorcism.

"If a God-fearing member of Parliament is threatened with eternal damnation for voting in favour of gay marriage, this is clearly as much of a contempt as any other threat to an MP - more in some instances," she said in a statement.

Ms Patten said since Kevin Rudd became prime minister, the number of MPs in the Parliamentary Christian Fellowship had risen from 75 to 84.

"This is way out of proportion to the 9 per cent of the Australian public who claim to be committed weekly worshippers," she said.

"We are a secular society increasingly being run by religious agendas."