After the military took Mr. Mugabe into custody, ZANU-PF expelled him as its leader on Sunday. But Mr. Mugabe stunned the nation that evening with a televised address in which he refused to step down as president. Pressure from within the country and from abroad had been building on Mr. Mugabe to resign, but observers had warned that the country might have to brace itself for lengthy impeachment proceedings.

The motion of impeachment introduced on Tuesday alleged, among other things, that Mr. Mugabe had violated the Constitution; that he had allowed his wife to usurp power; and that he was too old to fulfill his duties.

Earlier on Tuesday, Mr. Mnangagwa, whose firing led to the military intervention, broke his silence, urging the embattled leader to step down. “He should take heed of this clarion call by the people of Zimbabwe to resign so that the country can move forward and preserve his legacy,” Mr. Mnangagwa said.

Mr. Mnangagwa’s role as the likely successor to Mr. Mugabe has raised many concerns. He was accused of orchestrating the crackdown in the 1980s in which thousands of members of the Ndebele ethnic group were killed. He was also accused of being behind deadly violence in 2008 a bid to rig polls in favor of Mr. Mugabe, a claim he denies.

At least a semblance of legitimacy — especially for a government under Mr. Mnangagwa, who is known as the enforcer of some of Mr. Mugabe’s most ruthless policies — will be critical in gaining recognition from regional powers, Western governments and international lenders. Zimbabwe, which no longer has its own currency and perennially struggles to pay government workers, became a pariah in the West after the state-backed invasion of white-owned farms in the early 2000s.