At least 100 Chile fans reportedly stormed the media centre at the Maracana stadium ahead of the Spain against Chile game last night.
The fans attempted to access the stands inside the stadium without tickets and ended up in the area from where the media are covering the World Cup.
FIFA’s security staff was completely overrun as the Chile fans entered the press room, breaking several windows and plant pots at the entrance.
A few of the fans reportedly made it into the Maracana stadium by tearing some of the provisional walls in the media centre.
On their way, they reportedly broke TVs, tables, stole press material and took food from the journalists' canteen. 50 people were arrested and taken to local police stations.
A joint statement from FIFA and the local organising committee tried to play down the extent of the intrusion.
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."