FIFA says it rejected about 50 fake tickets for Thursday's opening World Cup match in Sao Paulo between host Brazil and Croatia.
FIFA marketing director Thierry Weil says the tickets were "an example of real professional fakes."
Weil held up a ticket that looked identical to an authentic ticket. He said it might even include the electronic chip - although a chip that would not work and would be detected when it was scanned entering the stadium.
He said some matches might be targeted more than other by counterfeit tickets, but did not say which matches they might be.
He said the 50 detected on Thursday must be only the tip of the iceberg and added there "must be far more."