Defeat for the ruling party would likely lead "to a denunciation of the election by the Mnangagwa administration and the potential for the military to intervene to secure power for ZANU-PF", the London-based BMI risk consultancy said.
Although the polls went off peacefully, several water cannon trucks patrolled outside the central Harare headquarters of the MDC as its supporters danced in jubilation across the streets.
European Union observers questioned the conduct of the presidential and parliamentary poll, Zimbabwe's first since Robert Mugabe was forced to resign following a de facto coup in November after almost 40 years in power.
Parliamentary results showed 109 seats for incumbent President Mnangagwa's Zanu-PF, and the opposition Movement for Democratic Change (MDC) with just 41.
Though some results in local races have been announced, she said full and official results might not be announced until the weekend.
Chigumba said the commission had until Saturday to announce results according to the law, but would have no problem to announce all results as long as they had verified them.
The observers expressed concern about delays in releasing the results of the presidential contest.
Former Liberian leader Ellen Johnson Sirleaf, the lead observer of a U.S. monitoring mission, said: "The more the presidential vote is delayed, the more it calls into question the population's confidence in the election process".
A presidential candidate needs more than 50% of the vote to win outright.
"We promise to invest in more business deals with Zimbabwe since it's a peaceful, democratic country".
"We are however seriously concerned about evidence of interference. there is a deliberate delay in announcing the results".
The EU mission said it found an "improved political climate, but (an) un-level playing field" after Zimbabwe's first polls since the military ousted longtime ruler Robert Mugabe previous year.
Even before the violence, European Union observers questioned the conduct of the presidential and parliamentary vote, the first since Mugabe's forced resignation after almost 40 years in charge of the southern African nation.
Blood could be seen on the streets of Zimbabwe after police used live ammunition on civilians and protesters who accused the electoral commission of rigging results.
Watching the results trickling in from the Zimbabwe Electoral Commission (Zec) in the early hours, it became apparent that the ruling Zanu-PF would have a majority in parliament.
Its population of 13 million is struggling amid shortages of foreign currency, unemployment above 80 per cent and lack of foreign investment.
Last night MDC principal Tendai Biti accused ZEC, which has been controlled by Zanu-PF for 38 years, of withholding election results, suggesting poll rigging might be at play.