"From what I saw, at least four people have been shot and killed," Adrian Pracon, a participant at the event told the Varden newspaper.
The gunman turned up at the event saying he wanted to ensure security there following the deadly explosion in the Oslo city centre earlier Friday, TV2 television reported, citing witnesses.
NRK reported that police had arrested the gunmen, but police have neither confirmed the arrest nor released any details on casualties.
Police did say that Prime Minister Jens Stoltenberg had been scheduled to attend the gathering, a summer camp on Utoeya, an island just outside Oslo. He was due to give a speech on Saturday morning to the 560 people attending the gathering.
The shootings came shortly after a deadly blast ripped through government buildings in Oslo, killing at least two people and wounding 15 according to media reports.