0335 GMT November 21, 2018
The country's first election since Robert Mugabe's ousting has been marred by rigging allegations, the army opening fire on opposition demonstrators killing six people, and accusations of a security crackdown on opposition supporters, AFP reported.
The MDC has until Friday to lodge its suit after Mnangagwa – formerly a close ally of Mugabe – won 50.8 percent in last week's vote, just scraping in above the 50 percent run-off threshold.
"Those results represent a total negation of the will of the people," MDC lawyer Thabani Mpofu told reporters in Harare. "The election results made by ZEC (Zimbabwe Electoral Commission) will be challenged."
Under Zimbabwean law, the deadline for filing the challenge is Friday. The Constitutional Court must rule on the petition within 14 days, delaying Mnangagwa's planned inauguration.
"ZEC's figures in our view grossly mathematically fail to tally," said Mpofu. "We are convinced once this has been placed before a court, there will only be one outcome."
He gave no details about any evidence of alleged fraud, promising instead to reveal "a secret weapon" in court.
Mnangagwa has vowed to protect rights in the "new Zimbabwe" after Mugabe's 37-year repressive rule but the MDC and civil action groups allege that authorities have unleashed a brutal post-election crackdown on opponents.
Senior MDC figure Tendai Biti was arrested as he tried to flee to neighboring Zambia to seek asylum early Wednesday, his lawyer said, allegedly on charges of inciting violence over the disputed vote count.
Biti last week claimed victory for the MDC party, triggering celebrations the day before soldiers opened fire on the protestors and before the final presidential results were announced.
"He was arrested at the Zambian border," Biti's lawyer Nqobizitha Mlilo told AFP, adding by text message that he was trying to seek asylum.