Norwegian television showed images of similar gatherings taking place in other cities across the country after a call for people to show solidarity with those killed in Friday's bombing and mass shooting.
"Tonight the streets are filled with love," Crown Prince Haakon told the vast crowd massed on the banks of the Norwegian capital's fjord after the car bombing of ministries and mass shooting of Labour Party youths on Utoeya island.
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"Those who were in the government district and on Utoeya were targets for terror. But it has affected us all," he said to applause.
Prime Minister Jens Stoltenberg then addressed the crowd, in a city of 600,000 people, saying: "Evil can kill a person but it cannot kill a people."
The mayor of Oslo, Fabian Stang, said: "We will punish the guilty. The punishment will be more generosity, more tolerance, more democracy."
Central Oslo streets were closed to traffic because of the vigil, which had originally been planned as a "flower march" but it was decided that people should stay in one place because of the large numbers turning up.
"We came out of solidarity, to all be together and share our pain," said Tone Mari Steinmoen, 36. "This is a time of important communion for our country."
"We're here to show that we're an open-spirited and respectful society," said Roy Kvatningen, 37, who came with his six-year-old daughter.
"And to support the victims," added his friend, Ger.
The crowd is united in grief, but there is no visible sign of anger towards the self-confessed perpetrator of the worst massacre in Norway since World War II, Anders Behring Breivik.
"We have no feelings about him. He's not our concern, we're here for our country, for the victims, for their families, not for him," said Benedicte Larodd, 26.
A police spokeswoman told AFP there was no official estimate for the turnout but described the gathering as "gigantic."
A policeman on the scene said there were about 100,000 people present, while Norwegian media put the figure at 150,000.
In any event, the gathering is the largest in Oslo in living memory.
The largely youthful crowd repeatedly raised their flower-bearing hands in the air, while a singer sang the Norwegian anti-Nazi hymn "For Youth" at the end of a short commemorative concert.
As the crowd dispersed before dark, they left a sea of bouquets on the ground, with dozens more flowers being laid on monuments around the grief-stricken city.
