The right-wing extremist had accused the singer of Children Of The Rainbow, Lillebjoern Nilsen, of being a Marxist who sought to brainwash children.
Nilsen led the crowd on Thursday in singing the song on Youngstorget Square, close to the courthouse.
Inside, the trial heard testimony from survivors of the Oslo bomb attack, BBC reports.
“It’s we who win,” said guitar-strumming folk singer Lillebjørn Nilsen as he led the mass singalong and watched the crowd sway gently in the rain. Many held roses above their heads, and some wept, The Guardian reports.