The celebrations opened on Thursday with a solemn event in the country's parliament, or Storting, led by the parliament's president Olemic Thommessen and attended by the country's Royal Family (including 10-year-old Princess Ingrid Alexandra in her first official visit to parliament), Prime Minister Erna Solberg, and representatives of the parliaments of Denmark and Sweden.
"Over 200 years we have built a good society and the celebration of the Constitution has indeed become a festival," Thommessen said in his speech, before warning Norway's people that they must fight to maintain the country's traditions of democracy and decency.
"Complacency is a dangerous servant and democracy is a fragile edifice," he said. "It must be maintained and if it is to continue we find good solutions in keeping with the times."
"With the present day we begin the next 200 years of Norwegian democracy," he concluded.
The ceremony was shown on a giant screen to the crowds waiting outside the parliament.
"It was great and atmospheric," Solberg told Aftenposten after the ceremony. "This day marks the birth of modern Norway. It's about the Constitution text, but just as important is the basis for Norway becoming a free and independent country in 1905."
Siv Jensen, the leader of the Progress Party, the coalition partner of Solberg's Conservative Party, said that the festivities should remind Norwegians to appreciate the things they take for granted.
"We have a constitution that so clearly empowers the individual rights and live in a democracy where we are so thoroughly respected. This is something we should think of when we regularly see pictures from other countries where oppression is commonplace," she said.
After the ceremony, Solberg, Jensen, and other top ministers served cake to the crowd outside the parliament, before a concert on the square.
The ceremony was brightened up by some 400 mayors from all over Norway, many dressed up in their tradition local costume, or bunad, who arrived in Oslo on Wednesday to enjoy a dinner hosted by Oslo Mayor Fabian Stang in the City Hall.
On Saturday, the celebrations in Oslo will start early, with marching bands battering drums from 7am in the morning, as other Oslo residents return from events held on Friday night at clubs and bars across the city.
The most glamorous invite to get for Friday night is the one to the annual May 16 bash hosted by the eccentric hotel billionaire Petter Stordalen, who is expected to wave to his guests from the balcony of a giant replica of the Royal Palace built in the gardens of his Choice Hotel Group in Frogner.
On Saturday morning, there will also be choral music outside the city's main churches, and from about 8am, a succession of ceremonies at graves and statues of famous Norwegians — including Henrik Ibsen, who is buried at Oslo's Vår Frelsers graveyard.
Then, from 10am to 1pm, children's parades will make their way from Festningsplassen near the Akerhus fortress down towards the parliament, cheered by tens of thousands of Norwegians lining the streets, many in full bunad.
At the Royal Palace Square, another crowd will gather to watch the Royal Family waving to the passing procession from the palace balcony. After a break for lunch, the festivities will kick off again with a free concert, with most Norwegians slowly drifting off to private parties held across the city, or continuing their revelling in the city streets.