Bulawayo - Zimbabwean President Robert Mugabe openly pilloried his deputy and possible successor on Saturday, saying he might sack him, in a combative speech at a rally where his wife Grace was booed by some in the crowd.The 93-year-old leader's remarks exposed tensions in the ruling Zanu-PF party over who stands to take power after him, an event only expected when Mugabe dies but a generational change likely to prompt bitter battles.Vice President Emmerson Mnangagwa, nicknamed "the Crocodile", is one of the top candidates.But Grace Mugabe - 41 years younger than her husband - has become increasingly active in public life in what many say is a political grooming process to help her eventually take the top job.Mnangagwa has been accused by Mugabe loyalists of undermining the president and of fanning factionalism in Zanu-PF, which has already named the incumbent as its candidate for next year's presidential election.Mnangagwa, 75, was appointed vice-president in 2014 and has been widely touted as the obvious successor to Mugabe, who has led the country for 37 years.But a visibly incensed Mugabe unleashed wrath at his vice-president, sitting nearby, when speaking in Shona in the opposition stronghold city of Bulawayo."We are denigrated and insulted in the name of Mnangagwa. Did I make a mistake in appointing him as my deputy?" Mugabe asked. 'You know nothing' "If I made a mistake by appointing Mnangagwa... tell me. I will drop him as early as tomorrow. We are not afraid of anyone. We can decide even here," he said."If it has come to this, it is time we make a final decision," he added, telling Mnangagwa and his supporters they were free to leave Zanu-PF."You can go ahead and form your party because we honestly cannot have this. We cannot be insulted on a daily basis."Some rallygoers heckled Grace Mugabe, chanting at her "you know nothing" and "you are too junior" as the first lady spoke before a section of the crowd that broke into a popular local song "oyenzayo siyaizonda" which translates to "we hate what you do."She shouted back at the hecklers: "If you have been paid to boo me, boo, go ahead...I don't care, I am powerful."She then repeated her ambition to be appointed her husband's deputy."Even if I become vice president, is there anything wrong with that?"Mnangagwa was last month stripped of his role as justice minister, in a cabinet reshuffle widely considered part of a campaign to reduce his powers and quash opposition within the government.