Zimbabwe's opposition leader has said President Emmerson Mnangagwa's election victory is a "coup against [the people's] will".
Nelson Chamisa repeated his claim that the results announced late on Thursday night were fake, and said he had won Monday's presidential poll.
The electoral commission said "no skulduggery" was involved in the vote tally.
This was the first election since long-term leader Robert Mugabe was ousted.
Earlier, in a tweet, Mr Mnangagwa urged Zimbabweans to unite to create "a new beginning" after a vote that was intended to set Zimbabwe on a new path following years of repressive rule.
Addressing journalists on Friday afternoon, Mr Chamisa called on his rival not to accept "corrupted results". He added that his MDC Alliance would pursue all legal and constitutional avenues to challenge the official result.
Nelson Chamisa, 40, says the delay in announcing the results suggests wrongdoing
The opposition leader said he had evidence that ballot boxes were being transported in open trucks allowing them to be tampered with.
He questioned the discrepancies in the numbers of votes tallied in the parliamentary elections compared with the presidential poll. Both elections took place at the same time.
Mr Chamisa also said that his election agent was not allowed to verify the results on Thursday.
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 city after clashes between protesters and police on Wednesday left six people dead.
A police vehicle with a loudspeaker has been broadcasting the message: "Zimbabwe is open for business. We are here to protect you. Feel free to walk and open your business. All is well, fear not."
The city, which is seen as an opposition stronghold, is quieter than normal as people are digesting the outcome, correspondents say.
But there were celebrations in one part of Harare. The president's supporters in the suburb of Mbare took to the streets to welcome the news.
Mr Mnangagwa avoided a run-off by just 36,464 votes out of more than 4.8 million cast.
Official results show he took 50.8% of the vote to Mr Chamisa's 44.3%. The 21 other candidates took up the remainder.
At the last election, Mr Mugabe won 61% of the vote, with the opposition's main candidate, the late Morgan Tsvangirai, winning 34%.
The Zimbabwe Electoral Commission (Zec) finished announcing the presidential election results in the early hours of Friday morning, after days of waiting.
Zec acknowledged that the wait had caused anxiety, and international observers had earlier urged the commission to speed up the announcement. Under the constitution, the commission had until Saturday to declare the result.
The results of the parliamentary election were announced earlier in the week. They gave Mr Mnangagwa's Zanu-PF 144 seats; the MDC Alliance, which is made up of seven parties, 64 seats, and one seat to the National Patriotic Front, formed by Mugabe loyalists.
Although Zanu-PF won by a landslide, its majority has shrunk since the 2013 election when it obtained 160 seats and the MDC 49.
A respected Zimbabwean independent monitoring group has added weight to the official result.
The Zimbabwe Electoral Support Network said, according to its sample survey, that Mr Mnangagwa had probably won enough support to avoid a run-off, though it left room for some doubt.
Mr Mnangagwa has vowed to revitalise Zimbabwe's tattered economy after decades of international isolation under Mr Mugabe.
The country has suffered from rampant inflation and high levels of poverty. The unemployment rate last year was as high as 90%, according to Zimbabwe's biggest trade union.