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".
Riot police initially prevented the opposition MDC Alliance from holding a press conference in the capital and chased reporters away.
The authorities later apologised saying that they thought an unlawful public gathering was being organised.
President Mnangagwa tweeted that what the police tried to do had "no place in our society".
Police are patrolling the streets of the city after clashes between protesters and police on Wednesday left six people dead.