Facing terror with music, tens of thousands have gathered across Norway to sing a children's song that mass killer Anders Breivik has claimed is being used to brainwash young Norwegians.

The event was organised as an act of defiance against the right-wing fanatic as survivors gave tearful testimony in his trial for the July 22 bombing and shooting massacre that killed 77 people.

In the drizzling rain, 40,000 people converged at a central square in Oslo to sing the 1970's song Children of the Rainbow - a Norwegian version of American folk music singer Pete Seeger's Rainbow Race, Belfast Telegraph reported.