Some of the private security personnel hired by soccer's world governing body FIFA did not turn up for work at the Castelao stadium in the northeastern city of Fortaleza, where Brazil drew 0-0 against Mexico in front of a 60,342 crowd.
"FIFA had hired the number of security guards needed for the game, but for some reason that we don't know they didn't show up," said a spokeswoman for the Ministry of Justice, which is in charge of police enforcement and public security in Brazil.
FIFA and the Brazilian government decided to bring in public security forces due to the magnitude of the game, the World Cup local organising committee told Reuters in an e-mail.
The committee did not explain the no-show by private guards, but said security arrangements for other World Cup games would not be changed.
Brazil has deployed more than 150,000 police and troops to prevent anti-World Cup demonstrations from disrupting the world's biggest sporting event.
Some demonstrations across Brazil have turned violent, but most have involved only a few hundred people.
The incident in the Castelao stadium was the latest security flop at a World Cup venue in the care of FIFA.
On Sunday, thousands of fans missed the start of the Ecuador-Switzerland game in Brasilia because they had to wait in long lines to get into the stadium due to enhanced security checks at metal detectors that were relaxed as play began.
A Brazilian government source said security at the Brasilia venue was reinforced after Chile fans smuggled fireworks into the Pantanal arena in the western city of Cuiaba and set them off after their team's victory over Australia on Friday.
In another security-related incident, dozens of ticketless Argentina fans stormed the Maracana stadium in Rio de Janeiro on Sunday to watch their team play.