HARARE, Zimbabwe — President Robert Mugabe of Zimbabwe, the world’s oldest head of state, abruptly dismissed the country’s vice president on Monday, a move that positions Mr. Mugabe’s wife, Grace, to succeed him as president.

The vice president, Emmerson Mnangagwa, who retains strong support from the military, had been seen as a main rival to Mrs. Mugabe in the internal fight over who might succeed the president, who led his country to independence in 1980 and is the only leader most Zimbabweans have ever known.

“The vice president has consistently and persistently exhibited traits of disloyalty, disrespect, deceitfulness and unreliability,” Information Minister Simon Khaya Moyo said on Monday in explaining Mr. Mugabe’s decision. “It had become evident that his conduct in his discharge of his duties had become inconsistent with his official responsibilities.”

Mr. Mnangagwa’s critics had accused him of plotting to take charge of key state institutions and of forming a set of parallel institutions within the ruling party, known as ZANU-PF.

A precipitating event in his downfall might have been a youth rally on Saturday in Bulawayo, Zimbabwe’s second-largest city, at which Mr. Mnangagwa received rousing cheers from supporters. At the same rally, Mr. Mugabe and his wife made blistering attacks on Mr. Mnangagwa, accusing him of disloyalty.

“If he wants to form his party with those who support him, he must go ahead,” Mr. Mugabe told supporters. “We can’t have a party where we insult one another. I don’t like that!”

Mr. Mugabe said that “we are going to decide a lot” at a special party congress scheduled for December, but added: “I felt I should say it here because I am annoyed.”

Part of the divisions within the party are generational.

Grace Mugabe, 52, leads a faction known as the Generation of 40 — so called because it began with leaders in their 40s; Mr. Mnangagwa, 75, is a veteran of the struggle for independence for Zimbabwe, formerly Southern Rhodesia. Since independence, Mr. Mnangagwa has been a minister in charge of many portfolios, including state security, justice, rural housing and defense. He has also served as speaker of Parliament.

“This has been building for some time, and the Mnangagwa camp, in trying to fend off the attacks, has made key mistakes and underestimated the strength that Grace Mugabe and her Generation of 40 have been mustering,” said Stephen Chan, professor of world politics at the School of Oriental and African Studies at the University of London. “Relying on his bedrock of military support, it was in the end not enough to prevent President Mugabe from moving against him — almost certainly to allow his wife time and space to build a presidential constituency of her own.”

The next step, Professor Chan said, is to see whether Mr. Mnangagwa is expelled from the party. He retains significant support from the rank and file, so if he leaves the party it could lead to a reduced majority for whoever wins the presidency in 2018.

Rashweat Mkundu, a former director of the Media Institute of Southern Africa, called the move “an end to ZANU-PF as we know it,” with the once-independent party now essentially a vehicle for the president and his family.

“Mugabe will now have to watch for rivals on many fronts, and Zimbabwe’s future is more uncertain with security at higher risk than before,” he said.

Nicknamed the Crocodile because of his own reputation for political ruthlessness, Mr. Mnangagwa had been booted from power before. In 2004, he lost his post as the secretary for administration in the ZANU-PF, after being accused of openly angling for the post of vice president. His rival at the time, Joice Mujuru, became vice president and the favorite to succeed Mr. Mugabe.

But in 2014 Mrs. Mujuru was herself purged, accused of plotting a coup, performing witchcraft and wearing miniskirts, and Mr. Mnangagwa was appointed as her replacement. (Mrs. Mujuru now leads the opposition National People’s Party.)

Whether Mr. Mnangagwa will opt to join forces with opposition parties — like the Movement for Democratic Change led by Morgan Tsvangirai — remains to be seen.

But his allies, many of them fellow fighters from Zimbabwe’s armed struggle for independence, were quick to express outrage.

“We do not care if Mugabe uplifts his wife or not,” said one ally, Victor Matemadanda, the secretary general of the Zimbabwe National Liberation War Veterans Association, expressing the view that Mr. Mugabe’s grip on power has been seriously weakened.