That omission gave room for the electoral commission, which ZESN and the opposition have accused of bias, to manipulate the results in favour of Mnangagwa and the ZANU-PF party, Biti said.
The party has already won the majority seats in the parliamentary elections and would want to complete a clean sweep by claiming victory in the much-coveted presidential polls.
On Wednesday, the election commission announced that the ruling ZANU-PF had taken majority of the seats in parliament, winning 109 out of 210 seats with 41 going to the opposition MDC and 58 yet to be allocated.
In an indication of the growing tension, a crowd of about 100 MDC supporters gathered outside a Harare hotel where election results were being announced but police blocked the entrance to the building, a Reuters witness said.
MDC leader Chamisa has said outright that his own count shows he won the election, drawing government accusations of inciting violence.
He has repeatedly accused ZANU-PF and election authorities of trying to use a flawed electoral register and fixed ballot papers to steal the election.
In Harare, the contrast could not be starker with November, when hundreds of thousands filled the streets, hugging soldiers and celebrating their role in ousting 94-year-old Mugabe, the only leader Zimbabwe had known since independence in 1980.
Outside the main opposition alliance Movement for Democratic Change (MDC) headquarters in the capital, at least six riot police trucks stood by with nearby shops remaining closed and barely any traffic on the usually busy Nelson Mandela avenue.
"As we speak, it has been discovered that ZEC did not post V11 forms at 21% of the polling stations, and we are gravely concerned by their failure to do so", he said.
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Also in attendance were Commissioners from the National Electoral Commission of Zimbabwe, Heads and representatives of Diplomatic Missions in Harare, representatives of political parties, faith-based groups, members of the public, members of civil society, and the media fraternity.
The commission said it would release vote totals "sometime" on Thursday, even though it said most of the results "are here with us". Ibhetshu likaZulu secretary general Mbuso Fuzwayo said it was sad that selfish opposition leadership donated Matabeleland seats to Zanu PF. "Kunashe and Mangwendeza owe the people of Bulawayo an apology and their leaders who deployed them", Fuzwayo said.
Incumbent leader Mr Mnangagwa, 75, had promised a free and fair vote after a military coup overthrew his former boss, Mr Mugabe, ushering him into power previous year. If you combine the opposition votes they are more than that of Zanu PF candidates who won.
Quiet descended as people waited for election results expected later in the day.
MDC leader Nelson Chamisa, 40, has said the results were fraudulent.
A credible and peaceful vote was meant to end Zimbabwe's worldwide isolation and draw in foreign investment to revive the shattered economy.
At the press briefing, Zec also announced the last of the House of Assembly results, and the MDC Alliance's Elizabeth Java Tsvangirai and Susan Matsunga won the Glen View South and Mufakose constituencies respectively.