President Emmerson Mnangagwa says he believes former first lady Grace Mugabe had become mentally unstable towards the end of her husband's long rule.
In an interview with Britain's Financial Times, Zimbabwe's new president said the incident that convinced him of this was a rally held by Grace Mugabe in November, in which she called Mnangagwa a snake who should have his head crushed.
'She went berserk'
"I was being castigated there as a snake. And to deal with this snake you must crush the head. And this snake is Mnangagwa, we must crush the head, not beat the tail or the body," Mnangagwa recounted the first lady as saying.
The rally in question was held a day before Mnangagwa was sacked by Mugabe on November 6.
Mnangagwa did not attend the rally, but said he had listened to reports about it.
"She went berserk on that one. At that stage now I believed she was not mentally OK," Mnangagwa said.
He took over from Mugabe later in November after the army stepped in and the 93-year-old leader was forced to resign after nearly four decades in power.
The Zanu-PF youth wing organised a series of rallies last year in support both of Mugabe's presidency and Grace Mugabe's likely bid for higher office.
Despite the criticism, Mnangagwa attended most of the rallies, including one in August in which he claims he was deliberately poisoned by opponents within the party. He told the Financial Times his doctors believe he was given a dose of rare "hard metal arsenic toxin".
"They say it is colourless, it is tasteless, and the areas where it could be found are possibly two... Russia and Israel. So it's possible it came from Russia," he said.