Zimbabwe’s opposition says its candidate, Nelson Chamisa, has won Monday’s presidential election.
The MDC Alliance says the ruling Zanu-PF party is attempting to rig the vote to allow President Emmerson Mnangagwa to win, and the delay in releasing official results is unacceptable.
The election commission has said there has been no cheating and it needed time to collate the votes.
The polls were the first since long-serving ruler Robert Mugabe was ousted.
Speaking at a press conference in the capital, Harare, the MDC Alliance’s Tendai Biti said there was a clear attempt by Zanu-PF to interfere “with the people’s will”.
Zanu-PF, which has been in power since 1980, has been accused of rigging elections in the past to keep Mr Mugabe in office.
However, a party spokesman told the BBC he had “no clue” what Mr Biti was talking about.
Monday’s vote attracted a high turnout of 70percent and was monitored by international observers.
European Union and US election monitors have been allowed into the country for the first time in 16 years to assess whether the elections are free and fair.
Zimbabwe Electoral Commission (Zec) chairwoman Priscilla Chigumba said she was satisfied there was no rigging.
“We will not subvert [the people’s will],” she told journalists.
When will we know the official winner?
Zec has until Saturday to announce the results but Ms Chigumba said she expects the announcement to be made well within that deadline.
Observers say the race between Mr Mnangagwa’s Zanu-PF party, and Mr Chamisa’s MDC Alliance, is extremely tight.
Both men are among 23 candidates running for president.
Zec has announced some of the results in the parliamentary elections but says it needs time to pull together the figures for the presidential poll from across the country.
A presidential candidate needs more than 50percent of the vote to win outright. Otherwise, a run-off election will be held on 8 September.
What are foreign observers saying?
European Union chief observer Elmar Brok said it was too soon to make a judgement but voting had been “very smooth” in some areas and “totally disorganised” in other areas, Reuters news agency quotes him as saying.
“There are shortcomings that we have to check. We don’t know yet whether it was a pattern or whether it was a question of bad organisation in certain polling stations,” Mr Brok told the AFP news agency.
As well as worries about the voters’ roll, the opposition has expressed concern over the security of ballot papers and voter intimidation in mainly rural areas.
Liberia’s former President Ellen Johnson Sirleaf, who was monitoring the poll on behalf of the US-based National Democratic Institute, told the BBC that Monday’s long queues showed Zimbabweans were enthusiastic about voting, without any kind of repression.
“I think this is an exciting moment for Zimbabweans to change the course of their country through their votes,” she told the BBC.