The USS New Mexico submarine arrived in Tønsnes municipal harbor, in Tromsø city, on Monday.
The Norwegian government imposed a no-fly zone over the port, strictly guarding access to the harbor area under tight security.
The military guarding of the port will only last for as long as the submarine stays, said the army.
People gathered at the port to protest the submarine's arrival and the government's failure to "take adequate preventive measures to avert a possible nuclear catastrophe.”
They held banners that read, “US Army out of Norway” and “No nukes in Tromsø.”
The port in Tønsnes normally serves civilian vessels, including the oil and gas supply activities offshore northern Norway.
The leader of Norway for Peace said Norwegians “fear that Tromsø will become a bomb target if there is a war.”
“The contingency plan doesn't address whether there are nuclear weapons on board the vessel,” said Ingrid Schanke.
Protesters also say the city, home to 76,000 residents, is in danger as the municipality doesn't have the expertise or equipment to handle a possible accident involving radioactive leakage.
"I have shown up to show resistance to what is happening here now, because if an accident should happen, the consequences haven't been taken into account," said a protester.