Mnangagwa survived an attempt on his life last month when a grenade was thrown at the stage during a campaign rally.
In a measured response, he said he suspected dissidents from his own party were behind the attack.
Those who know him say his reaction to the attack was typical of a man trained as a lawyer who leads with a quiet authority that surprises many who meet the feared former intelligence chief.
“If I had to describe him, the first word that comes to mind is ‘calm’,” a senior Western diplomat in Harare said.
“He has control of a room without having to shout. A little look or a wry laugh is enough to tell people his mind.”
Son of farmers, Mnangagwa learnt his politics in prison in the 1960s after being sentenced to death for sabotage by British authorities.
He was captured while in one of the earliest guerrilla units fighting white colonial rule in what was then Rhodesia. Only 19, he was spared the noose by a law prohibiting the execution of convicts under 21.
After a decade in prison, often sharing a cell with Mugabe, Mnangagwa became personal assistant to the leader of the liberation struggle, and went on to head the guerrilla movement’s feared internal security bureau.
Mnangagwa backed Mugabe’s economic nationalism, especially a drive to force foreign firms to hand majority stakes to local blacks, suggesting he may not be the pro-market pragmatist many investors were hoping for.
He has been in every administration since independence, holding posts as varied as Minister of State Security, Defence and Finance, as well as Speaker of Parliament.
Most controversially, Mnangagwa was in charge of internal security in the mid-1980s when Mugabe deployed a North Korean-trained brigade against rebels loyal to his rival Joshua Nkomo.
Rights groups say 20 000 civilians, mostly from the Ndebele tribe, were killed in what has become known as the massacres of Gukurahundi, meaning “the early rain which washes away the chaff before the spring rains”.
Mnangagwa’s role remains shrouded in mystery. Always a political operator, he trained as a communist guerrilla in China in the 1960s and stayed in the shadows behind Mugabe.
Mnangagwa denies responsibility but prior to the killings he said the government needed to bring “DDT” to rid itself of “cockroaches”.
“Mnangagwa knows only too well about his genocidal involvement in gukurahundi,” Jonathan Moyo said.
“He must be haunted by “cockroaches” & “DDT”; the screams of torture victims and the tears of the dead.”- Reuters