According to reports, the man poured liquid over himself before igniting it, and then sought to get inside the courthouse in Oslo.
He was stopped by officials when he reached a temporary security tent set up to scan everyone entering the building during the 10-week trial.
Right-wing extremist Anders Behring Breivik is standing trial for mass murder and terrorism after he shot dead 77 people and carried out a bombing that killed eight in Norway last July.
Kjell Kverme, from the Oslo police, said the man had been taken to hospital with serious injuries.
Kverme added that the man is understood to be Norwegian.