Around 100 fans broke into the Maracana stadium in Rio de Janeiro ahead of Chile's Group B clash with Spain.
The fans -- apparently mostly Chilean fans without tickets trying to get into the stadium -- broke through temporary walls and sprinted into the FIFA media centre, causing some damage.
Security guards were slow to respond and small in number -- one guard grabbed a Chile fan by the neck.
''I was the lone guy standing out there [near entry to press centre],'' security guard Diego Goncalves said. ''All of a sudden they knocked down the fence and just pushed their way through.''
Asked how many guards should have been watching the entryway where the fans burst through, Goncalves said "about 20."
ESPN FC correspondent Gabriele Marcotti, at the stadium to report on the match, wrote on his Twitter feed: "Chile fans breach security... Try to break into ground ... Security overwhelmed."
Sitdown protest in belly of Maracana... This is nuts ... Waiting for tropa do choque pic.twitter.com/DraoJ3QAGP- Gabriele Marcotti (@Marcotti) June 18, 2014
He added: "No cops down here, maybe 40 Chileans a dozen stewards [and plenty media]."
ESPN FC's Raphael Honigstein, meanwhile, tweeted: "Chile fans have just stormed the media centre, trying to get into the ground."
That was my seat... pic.twitter.com/njawG7hCjU- Raphael Honigstein (@honigstein) June 18, 2014
Miguel Delaney, also at the stadium to report on the game for ESPN FC, wrote on his Twitter account: "Loads of Chilean fans have just stormed the media centre, running around. A makeshift wall has been knocked down. Oh.
"A few people seem to have got hurt, more seriously, including a middle-aged fan coming to the stairs beside us.
"It wasn't a protest. It was fans without tickets, who were already gathering outside gates five hours before kick-off."
Door smashed at front of Maracana media centre. Plant pot also thrown pic.twitter.com/mTL0cxWplQ- Miguel Delaney (@MiguelDelaney) June 18, 2014
A joint statement from FIFA and the local organising committee said that "at least 85 intruders" had forced their way into the stadium but that none had made it to the seating area for the match.
The statement said: "Ahead of the Spain versus Chile match at the Maracana, a group of individuals without tickets violently forced entry into the stadium, breaking fences and overrunning security. They were contained by the security and did not make it to the seats.
"The situation was brought under control quickly and at least 85 intruders were detained according the military police of Rio. The organisers of the FIFA World Cup condemn these acts of violence and we will communicate further information and measures to be taken in due course."
Information from the Press Association and Associated Press was used in this report.