HARARE (Reuters) - Zimbabwe vice president Emmerson Mnangagwa said late on Thursday he had been hospitalized in August because he had been poisoned, raising the political temperature in the fight to succeed 93-year-old President Robert Mugabe. Mnangagwa, a former intelligence chief nicknamed  the Crocodile , is the leading candidate to succeed Mugabe, the only leader Zimbabwe has known since independence in 1980. He did not say who he believed was responsible for trying to kill him, and his main rival for the top job, First Lady Grace Mugabe, swiftly denied having anything to do with it. Hours earlier, she told a public gathering at a nearby hotel on Thursday night that Mnangagwa and his allies were plotting a power grab that was tantamount to a coup, according to an audio recording of her speech. Mnangagwa was airlifted to Johannesburg after falling ill in August.  At a news conference late on Thursday open only to state media, he said doctors had concluded that poisoning was to blame for his illness, and not inadvertent food poisoning.  The medical doctors who attended to me ruled out food poisoning but confirmed that indeed poisoning had occurred and investigations were in progress,  Mnangagwa said, reading from a statement. He provided no further details or proof. Mnangagwa, 75, became vice-president in 2014, putting him at the front of the pack to succeed Mugabe. However over the last 18 months he has met fierce opposition from Grace and a faction of the ruling party backing her. The first lady denied having anything to do with his illness and accused him of lying about it to get public sympathy.  Why should I kill Mnangagwa? Who is Mnangagwa on this earth?  Grace Mugabe said in footage aired on Friday on state television.  Killing someone who was given a job by my husband? That is nonsensical.  Veterans of Zimbabwe s 1970s independence war have publicly backed Mnangagwa to step into Mugabe s shoes and last year  described the nonagenarian ruler as a dictator, a jolting rebuke that laid bare the fissures within the ruling ZANU-PF party. In her hotel address on Thursday evening, Grace said Mnangagwa was plotting to force ZANU-PF to support him as Mugabe s successor - a scheme she said must be resisted.  We are being threatened day and night that if this one does not become president, we will kill you,  she said without stating the origin of the threats.  No, we will not bow down to that pressure, never. You will have to arrest all of us and throw us in prison before you can rule.  According to a trove of hundreds of documents from inside Zimbabwe s Central Intelligence Organization (CIO) reviewed by Reuters in September, Mnangagwa and other political players have been positioning themselves for the day Mugabe either steps down or dies, with the tacit backing of some of Zimbabwe s military and former colonial power Britain. The ruling party s next leadership congress is due in 2019 and Mugabe, who plans to context next year s elections, has said he is staying put for now. 