The handling of the election in Zimbabwe, where the votes are being counted, has drawn criticism from election observers from southern African nations.
The calm voting, according to the regional organisation known as SADC, fell short of some local laws and its own rules.
More than 40 poll watchers were detained late on Wednesday as they attempted to calculate their own vote total to compare with the official result.
Rights groups have denounced the arrests.
In the midst of high unemployment and rising inflation, President Emmerson Mnangagwa is running for re-election.
Nelson Chamisa of the opposition Citizens Coalition for Change is his main rival.