OSLO (Reuters) - Thousands of Norwegians will take to the streets of Oslo on Thursday to sing a children's song calling for peace and fraternity, in a protest against mass killer Anders Behring Breivik who has called it Marxist brainwashing.

They plan to sing arm-in-arm a few blocks from the courthouse where Breivik is on trial for the killings of 77 people in a gun and bomb rampage last year.

"I grew up with this song and have sung it to my child," said Lill Hjoennevaag, one of the organizers of the demonstration.

"Everybody I know feels strongly about this song and we need to take it back," she told public broadcaster NRK.

Lillebjoern Nilsen's "Children of the Rainbow", a Norwegian rendition of American folk singer Pete Seeger's 1971 "My Rainbow Race", is a popular song in Norway.

"Breivik has used it as an example of brainwashing, but it is rather an example of the opposite," said Christine Bar, another organizer, who launched the event on Facebook.

"We think it represents diversity, and it stands for the community we have chosen to live in, and which Breivik and similar people want to tear down," she added.

Breivik set off a car bomb, killing eight people, then gunned down 69 people, mostly teenagers, at a youth summer camp organized by the ruling Labour Party on July 22.

In court on Friday he attacked Norway's schools and in particular Nilsen.

"He is a good example of a Marxist who infiltrated the cultural sector; he writes music that is used to brainwash children," Breivik said.

He admits the killings and has described his attacks in shocking detail, but pleads not guilty, claiming the right to protect Norway from multiculturalism and Muslim immigration.

He said the Labour Party was his chief target because it had promoted mass immigration, and that its youth camp was political indoctrination with the aim of brainwashing young activists.

He expressed regrets he was unable to complete his plan of beheading former Prime Minister Gro Harlem Brundtland, known as the "mother of the country", and said he had hoped to kill many more.

On Wednesday he railed at psychiatrists who diagnosed him as psychotic, saying their report contained "evil, fictional inventions".

Selected lyrics, as translated by Reuters, follow:

A sky full of stars

Blue ocean far as you see

An earth where flowers grow.

Can you wish for more?

Together we shall live

Every sister, every brother

Small children of the rainbow

And a fertile soil

....

But tell all the children

Tell your father, tell your mother

This is our last chance

To share one hope, one world

(Reporting by Balazs Koranyi; additional reporting by Victoria Klesty; Editing by Andrew Roche and Alessandra Rizzo)