A blast was heard across Johannesburg on Sunday followed by a plume of smoke, visible from the Soccer City where World Cup matches are being played, but South African police said it was a routine test.
The loud boom heard by reporters was caused by testing at an explosives factory in the western suburbs of Johannesburg, some distance from the stadium, police spokesman Eugene Opperman said.
"It was controlled explosion. They are a registered company so they are allowed to do their own business and testing," he told said.
No match was underway at the Soccer City when the boom was heard shortly after midday, although Brazil is due to face Ivory Coast later.
Police added in a statement there had been no injuries or damage from the explosion.