Staff at Forsmark 1 received the phone call at around 9am, according to trade union newspaper Sekotidningen’s website.
The threat has not affected operations but all maintenance staff have been evacuated, according to Sekotidningen. Police were reported to be searching the plant.
Forsmark spokespeople were at lunchtime on Wednesday still not giving details of the threat or of the measures being taken.
Claes-Inge Andersson, communications director at the plant, was directing inquiries to the police.
Christer Nordström, spokesman for Uppsala Police, was giving little away.
“I can just confirm that there is a threat, but I cannot give any details of the content,” he said.
“We have begun a major operation in conjunction with the power plant,” he added.
Road blocks have been set up around the plant, but Nordström would not comment on the reports of an evacuation or on whether the threat had affected electricity generation.
While police said they were taking the situation “very seriously”, they did not consider there to be an imminent threat.
“Our judgment is that there is currently no impending danger for the power station,” said Nordström.
Uppsala Police have requested help from Stockholm Police’s bomb squad.
“We have also informed the Security Police [Säpo] and the County Administrative Board,” Nordström said.
Henrik Ohlsson, of the emergency planning unit at the Uppsala County Administrative Board, said that his unit was not currently involved in the incident.
“In the event that something happens, if there is an accident, we have responsibility for the emergency services, but this is currently a question entirely for the police,” he said.
The threat to Forsmark has led to the alert level being raised at the Ringhals plant, in south-western Sweden, according to TV4 Halland.
No direct threat has been received by Ringhals, and officials at the plant said they would not comment on the measures they were taking.