“We sang the chorus, ‘Singing Norway, Norway out of Nato'. It was a hit,” he joked to members of Norway's Labour Party youth as they arrived on the island of Utøya for the first summer camp since the anti-Islamic extremist Anders Behring Breivik unleashed his gun massacre there in 2011.
Stoltenberg said he had first attended the camp in 1972, at a time he said was “one of the absolute bottom points of the AUF's history”.
The Labour Party had just lost its campaign to bring Norway into the European Union, after Norway voted ‘no' in a referendum, and many moderate members had left the AUF, as the youth group is called, leaving it in the hands of hard-left militants.
“It was really just some weird people that were left, and some confused children of party loyalists, like me,” he said.
This year, a record 1,000 people signed up for this year's camp on Utøya, and on Thursday the first of them arrived to erect their tents.
Breivik's attack led to the death of some 69 people, most of them teenagers and many killed with shots to the head.
“The horror that happened in 2011. It's going to be a part of Utøya history for all times to come,” Stoltenberg told Norway's NTB news wire.
While Stoltenberg was head of the AUF leader from 1985 to 1989, it changed it policy to support Nato.
“I am glad that AUF took the standpoint they took back in the 80s, but it is not NATO which is on the agenda here,” the said in his talk.
In his early teens, Stoltenberg was influenced by his sister Camilla, a member of the then Marxist-Leninist group Red Youth, and participated in protest rallies against the Vietnam War.
At the time that he led the AUF, he had regular contacts with a Soviet diplomat, who later turned out to be a KGB agent.
He served as Norway's Prime Minister from 2005 to 2013.