Controlled explosion near Soccer City stadium
The loud boom heard by Reuters 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 Reuters.
No match was underway at the Soccer City when the boom was heard shortly after midday, although Brazil is due to face Ivory Coast.
Police added in a statement there had been no injuries or damage from the explosion.