Having been on the throne since 1991, King Harald is Europe’s oldest head of state.
Crown Prince Haakon, 53, the successor to the Norwegian throne, has been serving as regent while his father is on leave.
King Harald has been treated in recent weeks with cortisone for haemolytic anaemia, a condition that causes the number of red blood cells in the body to be reduced, and leads to fatigue and shortness of breath.
There was a sombre mood in Oslo as Norwegians followed the comings and goings of the royal family. Crowds milled around in Palace Square throughout the day as Norwegian public TV scrapped its schedule to cover the story.
Although no-one in the royal family spoke, the message was clear that it had become an important moment to visit the king.
Among those visiting the hospital were Crown Princess Mette-Marit who arrived with Haakon, as well as her son Marius Borg Høiby, who is under house arrest during his appeal against a conviction for rape.
King Harald’s sister, Princess Astrid, also visited the hospital, along with Haakon and Mette-Marit’s children Princess Ingrid Alexandra and Prince Sverre Magnus.
Prime Minister Jonas Gahr Støre cancelled a planned trip to Germany, where he was expected to meet Chancellor Friedrich Merz on Thursday.
“The whole nation is with us as we send warm thoughts to the king and the rest of the royal family,” he said in a statement.
