An apparent grenade attack has rocked Kigali, the Rwandan capital, just hours after the election commission announced Paul Kagame, the incumbent president, had won re-election. 

About 20 people were injured in Wednesday's explosion, according to witnesses. 

Al Jazeera's Andrew Simmons, reporting from Kigali, said that explosion fits the pattern of previous grenade attacks in the city.

"If this is confirmed as a grenade attack it is in exactly the same area as a series of grenades earlier this year, the most recent in May," he said, adding that the previous violence was politically motivated.   

"It does appear to be linked to the election because all of these attacks were regarded as political, and the theory behind them was that military dissidents had been involved," he said.

Election observers had criticised the poll because the campaign had been deviod of "critical opposition voices", with the three other candidates standing in the poll linked to Kagame's party.