Norway's royal family gathered Thursday at King Harald V's bedside in hospital as the palace announced that the 89-year-old monarch's health condition was "extremely serious".
Queen Sonja and Harald's 53-year-old son Crown Prince Haakon, who is acting as regent, have cancelled all royal engagements and spent much of Thursday at his side at Oslo University Hospital along with other family members.
Haakon, his wife Mette-Marit and their two adult children, were seen leaving the hospital in the late afternoon.
Europe's oldest reigning monarch has been in hospital in Oslo since August 17 suffering from haemolytic anaemia, a condition caused by an abnormally rapid destruction of red blood cells which leads to fatigue and shortness of breath.
"His Majesty the King's health condition is extremely serious," the palace said, providing no specific details.
Mette-Marit's son from a previous relationship, Marius Borg Hoiby, who is under house arrest and wearing an ankle bracelet after a rape conviction in June that he has appealed, was also seen leaving the hospital after a more than four-hour visit, as was the king's sister.
Members of the public gathered outside the palace in central Oslo, some laying down bouquets of flowers.
Norwegian Prime Minister Jonas Gahr Store, who cancelled a work trip to Germany on Thursday following the update on the king's health, told AFP he was "deeply affected" by the news.
"I stand together with the entire country as we now send our warmest thoughts to the king and the rest of the royal family. I am confident that the king is being well taken care of, and that he is receiving good and compassionate treatment," he said.
Norway's bishops also cut short a study visit to Rome where they had met Pope Leo and were to return to Norway Thursday, according to Christian newspaper Vart Land.
Caroline Vagle, a royal reporter for the Se og Hor magazine, called Thursday's statement from the palace "dramatic".
"There is now a completely different atmosphere surrounding the King's illness than before. Over the years, the Royal Household has been very restrained in its communication about the King's health, even when he has been hospitalised or undergone treatment," she told AFP.
"There is also an entire nation that is deeply concerned and saddened. King Harald is a much-loved king."
The monarch was admitted to Oslo University Hospital due to fluid retention from cortisone treatment he was receiving.
The palace said on Sunday that he was receiving antibiotics for a bacterial infection in his blood and that he had responded to the treatment.
Harald, who ascended the Norwegian throne in 1991, has had multiple health problems in recent years, forcing him to have a pacemaker fitted and to scale back his official schedule.
But he has always ruled out abdicating, adamant that he took a lifelong oath before the Norwegian parliament.
In February, the king was hospitalised for an infection and dehydration in Tenerife, where he was on a private trip with Queen Sonja.
His wife, also 89, was briefly admitted to hospital in May due to heart problems.
Their mounting health issues have come on top of other royal woes.
Haakon's wife Mette-Marit is under scrutiny for her ties to the late US sex offender Jeffrey Epstein.
Documents published in January in the United States revealed her sustained and at times intimate correspondence with Epstein between 2011 and 2014, at a time when the American financier had already been convicted.
Mette-Marit's son from a relationship prior to her marriage to Haakon, Marius Borg Hoiby, is also in the spotlight as a result of his legal troubles.
He was sentenced in June to four years in prison for two rapes and 32 other charges, including violence against a former partner. He has appealed the verdict.
Suffering herself from a serious lung disease, Mette-Marit, 53, had to undergo a lung transplant in July.
The royals remain broadly popular with the Norwegian public, in large part to Harald's unifying status and his unassuming, modest lifestyle.
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