For supporters of Zimbabwe's opposition MDC Alliance party, this was a day they had been dreading - the moment their dream of election victory was crushed.
Three people were killed after soldiers moved into Harare on Wednesday, firing live rounds and beating protesters.
The electoral body's core staff is drawn from Zanu-PF and the Zimbabwe National Army.
Police have invoked a strict security act that forbids public gatherings.
State broadcaster ZBC reported that the electoral commission would announce the presidential results at 12:30 local time (10:30 GMT). "We are not going to accept defeat", he added.
"Are we in war?"
The opposition says it condemns violence in all its forms.
Armored vehicles have appeared on the streets in Harare.
European Union observers voiced concern about delays in releasing the results of the presidential contest, a two-horse race between Chamisa and Mnangagwa, head of the ruling ZANU-PF party.
The Zimbabwe Electoral Commission has announced it is not yet sure when the commission will announce the results of the presidential election.
Anti-riot police stand guard at gate of Rainbow Towers where election results were announced.
A man has been shot dead in the clashes between armed police operatives and protesters of the the opposition, Movement for Democratic Change (MDC) alliance.
The party's supporters today went on rampage, protesting against the results of Monday's harmonised elections which saw Zanu PF winning two thirds majority in parliament.
The assessments of Western and other observers, many of whom returned to Zimbabwe after being barred for almost two decades, are crucial in the possible lifting of worldwide sanctions on the southern African nation. "No amount of results manipulation will alter your will", he tweeted before the army was deployed.
Zimbabwe's much-criticised election authority declared that there had been no rigging of the country's election, after the opposition repeatedly alleged that the vote process was flawed. "If President Chamisa wins this election then the people of Zimbabwe will have their government".
ZEC chief Priscilla Chigumba said the presidential result may not be ready until Saturday. You voted for total Change in this past election!
But it said partial presidential results could be announced later Wednesday.
But the opposition says Zanu-PF rigged the election.
The commission had said there was no rigging or cheating in the first national election since the end of Robert Mugabe's almost four decade rule. Its population of 13 million is struggling amid shortages of foreign currency, unemployment above 80 percent and lack of foreign investment.
However, Douglas Mwonzora, a top MDC Alliance official, told the BBC's Andrew Harding that the endorsement on Sunday of their candidate by Mr Mugabe had cost the party votes.
Chamisa's MDC won in most urban centres, where it enjoys majority support.