More than 40,000 people packed the square near the courthouse holding the trial of anti-islamic killer Anders Behring Breivik to sing songs.
Inspired by a Facebook-organized protest, the crowd made the defiant statement in protest against the killer's far-right views that led him to kill eight people in a bomb attack in Oslo and a further 69 people on Utoya Island.
Roses, a symbol of the Labour youth party, were held and attached to the railings of the courthouse on the eighth day of a trial that has seen Breivik show no remorse for the killings, which he claims were aimed to stop a future war on multiculturalism.
Breivik's resolute claims that he is sane, along with unflinching testimony of how he carried out the murders, have rocked a country that thrives on its atmosphere of democracy.
However the protest reasserted the nation's strength in the face of extremism, as the crowd sand Children of the Rainbow, a 1970s song that celebrates multiculturalism and Breivik is known to hate, having decribed it as "Marxist propaganda".
Peter Solberg, a 46-year-old office worker, told Reuters: "I care about the people who died and whose family members died. This march is about them and about our Norway, not his [Breivik's] Norway."
Norwegian artist Lillebjoern Nilsen played the song - a Norwegian version of American folk music singer Pete Seeger's "My Rainbow Race."
The thousands sang the the Norwegian lyrics:
"A sky full of stars, blue sea as far as you can see
An earth where flowers grow, can you wish for more?
Together shall we live, every sister, brother, you and me
Young children of the rainbow, a fertile land and seashore."
Breivik, who was moved to tears by his own manifesto video warning of a culture war in Europe, has repeated that he regrets that he did not claim more victims in his attacks, which he does not accept legal culpability for.
Witness Harald Foesker, who was left need facial reconstruction following the bomb blast on July 22, 2011, gave his testimony on the attack, which left him "spitting teeth".
Reached at home in Beacon, N.Y., the 92-year-old Seeger told The Associated Press he had heard about the mass gathering from Nilsen, who called him Thursday morning.
"I said, 'Oh that's wonderful,'" Seeger said. "It's a tremendous honor, really. One of the greatest honors a songwriter could have is to have a song of their sung in another country."
Seeger and his music have been central in myriad social justice causes from civil rights to the environment. He sang out against the Vietnam War and more recently joined the Occupy Wall Street protest in Manhattan.
A majority vote of three of five judges will be required in the trial, with a verdict expected in mid-July. Breivik is charged with terrorism and premeditated murder for a bombing in Oslo's government district, killing eight, and a shooting attack at a political youth camp, killing 69.
If convicted he would face a maximum sentence of 21 years in prison, which he branded "pathetic", though sentences can be extended if a criminal is considered a menace to society. If declared insane by the court, he would be committed to psychiatric care. Both sides can appeal the ruling to a higher court.
In court Thursday, people who survived Breivik's car bomb in Oslo's government district gave emotional testimony as he listened expressionlessly.
Anne Helene Lund, 24, was just 7 meters (23 feet) from the explosion. Afterward, she was in a coma for a month and when she woke up she had lost her memory, unable to even remember the names of her parents.
"I studied political science for three years, now I have to relearn social studies at the junior high school level," she told the court.
Her father, Jan Henrik Lund, also took the stand. Fighting tears, he described his emotions at seeing his daughter with severe life-threatening brain injuries.
"It was like experiencing the worst and the best in the same moment," he said. "It was fantastic that she was alive, horrible that she was as injured as she was."
The trial continues.