Sustained gunfire erupted on Wednesday near the headquarters of Guinea-Bissau’s national election commission in the capital, a Reuters witness and two other residents said.
The coup-prone West African state held a presidential election on Sunday and provisional results are due to be announced on Thursday.
The vote pits incumbent President Umaro Sissoco Embaló against main challenger Fernando Dias, with both camps having claimed victory in the first round earlier this week.
One witness said people were seen fleeing as shots continued to ring out near the election commission around 1300 GMT.
It was not immediately clear who was involved in the shooting and whether there were any casualties.