The attack on the stand took place late Saturday afternoon. According to Göteborgs-Posten, the man spat the red liquid on the flag.
He was taken into custody and removed from the scene by guards and police, according to the newspaper.
Police spokesperson Emelie Lundqvist cannot confirm the information about police involvement.
"We haven't received any alarms," she tells TT.
"But we are stationed at the Book Fair. We can, in accordance with the Police Act, reject or remove a person from a place," says Lundqvist.
The police have no information about any arrests or whether there is any suspicion of a crime.
