England fans were targeted by Brazilian hooligans who threw an explosive device ahead of the match against Uruguay in Sao Paulo, according to Brazilian media.
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Globo has reported that 15 people were arrested after a hooded group attacked Three Lions supporters at two neighbouring bars in downtown Sao Paulo, close to the official FIFA Fan Fest.
One report said a number of England fans were taken to hospital for treatment, but Globo quoted a bar owner as saying no customer had been injured.
An eye-witness said: "The guys jumped on a bus to try to escape, and then the police went after them."
After the attack, police seized six rockets, a knuckle-duster, a dagger and other fireworks.
The British Consulate are aware of an incident, but said they are not aware of any casualties.
A statement read: "We are aware of an incident affecting fans around the area of the Fan Festival in Sao Paulo.
"We are working with the local authorities to establish the facts. No British nationals are believed to have been injured."