The impending and historic election in Zimbabwe has taken an extraordinary twist after a dramatic intervention by Robert Mugabe who savagely attacked the ruling party he had once led, and praised the main opposition candidate as the only viable option who can “return legitimate government to the country”.

The 94-year-old former president delivered a bitter invective against Zanu-PF saying: “I must say clearly I can’t vote for those who tormented me, no I can’t.”

He condemned the “evil and malicious characters” who had removed him from power and decried the military intervention which brought it about.

The former president, appearing in public for the first time since his overthrow last year, defended his wife Grace – accused of massive corruption – demanding critics “leave her alone”.

The polls on Monday, the first in 38 years without Mr Mugabe being in power, were being presented as a break from the dark past and the first steps towards repairing a fractured nation with a deeply divided society and an imploded economy.

But the sudden reappearance of the man who was, until recently, the longest serving head of state in Africa, has now added an uncertain new dynamic to that equation.

Mr Mugabe did not directly endorse the opposition MDC (Movement for Democratic Change) alliance. But there was undoubted approval for its leader who is challenging President Emmerson Mnangagwa, a former ally of Mr Mugabe who replaced him in last November’s coup.

Mr Mugabe said: “Nelson Chamisa seems to be doing well by his rallies. He wants a legitimate government to be in power and an end to one brought about by military intervention ... I would certainly wish to meet him if he wins.”

He himself, said the former president, will choose from the candidates for his vote, but it will not be Mr Mnangagwa, of the “18th November team” which carried out his overthrow.

There were immediate claims afterwards that Mr Mugabe’s wife, Grace, was the driving force behind the move.

The former first lady, who is accused by her critics of rampant corruption and plotting to run the country through her husband before his fall, is a bitter enemy of Mr Mnangagwa.

Ms Mugabe stood beside her husband, with an assistant holding an umbrella over her, as he spoke at a hastily and unexpectedly arranged press conference at Blue Roofs, the house in Harare where the couple had continued to stay after the president was stripped of office. At one point she asked Mr Mugabe to “speak up please”.

Mr Mugabe did very much want to speak out about the vilification of his 55-year-old wife by her many enemies, who call her “DisGrace” and “Gucci Grace”.

He said: “I do not accept the denunciation and vilification of my wife that is going on every day. Leave, leave, leave my wife alone. I want Grace to remain my Grace.”

Mr Mugabe, in a dark suit, white shirt and tie and pocket handkerchief in red, was alert and mostly cogent when answering questions. The military, who had helped crush dissent and kept Mr Mugabe in power over the decades, were a particular source of his anger. “I hope the choice of voting tomorrow will thrust away the military government and bring us back to constitutionality. Let tomorrow the voice of the people say never again shall we experience a period in which the army is used to thrust one person into power.”

Zimbabwe president Emmerson Mnangagwa survives 'assassination attempt' in campaign rally explosion

It was clear that Mr Mugabe expected Mr Chamisa to unseat that person, Mr Mnangagwa in the election. But it remains to be seen what effect the former leader’s endorsement will have on the MDC leader who is trailing Mr Mnangagwa by three points in the opinion polls.

Mr Chamosa, who once had his skull fractured by state security agents of the Mugabe regime, may gain from the former president’s supporters who resented his removal, but it may yet put off the 20 per cent who say they are still undecided.

Mr Mnangagwa, who had been careful not to say anything particularly critical of Mr Mugabe, may lose the support of the Zanu-PF old guard, but it may also provide an opportunity to put aside the baggage he had been carrying for his long alliance with Mr Mugabe.

On Saturday, in the last round of election rallies, Mr Chamisa was accompanied on stage by Eunice Sandi-Moyo, head of National Patriotic Front (NPF), which had been set up by Mugabe loyalists in March.

The membership were largely composed of the G40 or Generation 40 group in Zanu-PF who had hitched their wagons to the star of Grace Mugabe.

The MDC leader said: “Robert Mugabe is a citizen of Zimbabwe, former president, and the president of the first republic who is going to hand over to the second president of the republic – who is me, myself here present. Let me welcome the NPF here as the genuine Zanu-PF, it means we have the original Zanu-PF here, authentic and undiluted.”

The G40 have been blamed for the grenade attack in Bulawayo a month ago targeting Mr Mnangagwa which killed two people and injured more than 40 others.

No evidence has been produced so far to back the claim made by, among others, Mr Mnangagwa. A number of people were arrested after the attack, some have been freed.

Mr Chamisa had, in the past, been forced to deny rumours that he had received funding from the Mugabes. However, he had said: “We welcome every vote; Mugabe, we welcome your vote. We want a new dispensation, a fresh start.”