Nelson Chamisa, Zimbabwe's opposition leader, says President Emmerson Mnangagwa's election victory is a "coup against [the people's] will".
Chamisa believes that the results announced late on Thursday evening were fake, and said he had won Monday's presidential poll.
However, the electoral commission said no dishonest behaviour was involved in the vote tally.
This was the first election since long-term leader Robert Mugabe was ousted.
Addressing journalists on Friday afternoon, the opposition leader called on his rival not to accept "corrupted results".
Businesses in Zimbabwe's capital, Harare close its doors as they wait for the results from the presidential election.
Armed soldiers and police are on patrol in the capital city.
Three people were killed in the city on Wednesday in clashes between the security forces and supporters of opposition leader Nelson Chamisa.
The opposition leader says Monday's elections were rigged to give President Emmerson Mnangagwa victory.
The elections were the first since long-time ruler Robert Mugabe was ousted in November.