Jonas Gahr Støre, the leader of Norway's Labour party, said that Trump's statements on NATO and Russia were so “deeply disturbing" that it was no longer possible to keep to the convention of not commenting on other countries' elections.
“It is disturbing for Norway that someone who could become President of the US is putting in doubt the principles and obligations of Nato and opening up for reconsideration the doctrine of ‘one for all, all for one',” Støre told Norway's VG newspaper.
Støre joins French President François Hollande, Germany's Foreign Minister Frank-Walter Steinmeier, Sweden's Prime Minister Stefan Löfven and Italian Prime Minister Matteo Renzi in openly speaking out against a Trump presidency.
Hollande went so far as to say that the businessman and reality TV star's “excesses make you want to retch”, while Steinmeier on Friday called the US Republican presidential candidate a "hate preacher".
In nearby Sweden, Löfven has complained that Trump's campaign was based on “fear and division”.
Støre said that Trump's statements had been so extreme that it was no longer possible to remain silent.
“The usual thing has been to be reluctant to comment on other countries' elections, but this is an election with an impact far beyond the US,” he said. “There are both attitudes and opinions coming from Trump around politics which I think make it completely unnatural to sit idly still and watch.”
He also singled out Trump's attitude to Russia's annexation of the Crimean peninsula as another worrying sign.
“No country that wants to have safer borders in Europe is reassured by such statements,” he said.
He said he was also worried that the fight against climate change, and for the rights of women would be set back if Trump became president.
Støre later on Friday spoke at the summer camp of Norway's Labour Party youth movement, the AUF, on the island of Utøya, telling the 1,000 attendants that the world was now divided by those who viewed the world with hope, and those who felt only fear.
“This is a burning dividing line in our time; whether we approach the future with fear or hope,” he said.
“Fear is not wrong. Not inappropriate. Not shameful,” he continued. “Fear is a reality in many people's lives, and those of us who are engaged in politics must take it seriously, understand where the fear is coming from, and do something to counter it. The Labour movement came into being in order to overcome fear.”