HARARE, Zimbabwe — Frustrated by President Robert Mugabe’s refusal to step down, lawmakers in Zimbabwe formally began an effort on Tuesday to impeach him, using a constitutional process that has never before been tested and that could prolong the country’s political and economic uncertainty.

Members of the governing party, ZANU-PF, which has controlled Zimbabwe since independence, introduced a motion of impeachment that was seconded by its historical rival, the Movement for Democratic Change.

The motion was introduced after Parliament convened in a joint session. The next step is for Parliament to form a committee to investigate the motion’s allegations that Mr. Mugabe violated the Constitution; that he allowed his wife, Grace, to usurp power; and that, at 93, he is too old to fulfill his duties.

It was a striking sign of the consensus in the political class that Mr. Mugabe must go — a consensus that formed with astonishing speed after the military took Mr. Mugabe into custody last Wednesday, signaling an end to his 37-year rule.

ZANU-PF expelled Mr. Mugabe as its leader on Sunday, but Mr. Mugabe stunned the nation that evening with a televised address in which he refused to step down. Pressure from within the country and from abroad has been building on Mr. Mugabe to resign, but observers said on Tuesday that the country might have to brace itself for lengthy impeachment proceedings.

Greg Linington, a constitutional law expert at the University of Zimbabwe, said that the Constitution did not stipulate a time frame for impeachment, and that a thorough process could take weeks or months. Mr. Mugabe should be given the right to reply and time to prepare, Mr. Linington said.

“It’s important to get this right,” he said. “Suppose they don’t do it properly and later on Mugabe brings a court application challenging the way the procedure was done.”

According to Zimbabwe’s Constitution, a president can be removed for serious misconduct, violating the Constitution or “inability to perform the functions of the office because of physical or mental incapacity.” Committees must investigate and present evidence. Finally, Parliament can remove the president with a two-thirds vote in each of the two legislative chambers.

Emmerson Mnangagwa, the military-backed politician whose firing led to a military takeover of Zimbabwe and efforts to oust President Robert Mugabe, broke his silence on Tuesday, urging the embattled leader to step down.

Mr. Mnangagwa, the former vice president who has not been seen in public since leaving Zimbabwe for South Africa on Nov. 6, said he had refused the president’s invitation to return for talks to Harare, Zimbabwe’s capital. Despite having the backing of the powerful war veterans association and the military, Mr. Mnangagwa, 75, said he feared for his personal security in Zimbabwe.

“I told the president that the current political and constitutional crisis in the country is not a matter between him and myself but between the people of Zimbabwe and President Mugabe,” Mr. Mnangagwa said in a statement.

“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,” he added.

Mr. Mnangagwa’s words, as well as his continued absence, appeared to be part of an effort by his allies to distance him from last week’s military intervention and to portray it as a reflection of the popular will. The army stepped in two days after the president attempted to arrest the country’s top military commander, Gen. Constantino Chiwenga, a close ally of Mr. Mnangagwa.

At least a semblance of legitimacy — especially for a government under Mr. Mnangagwa, who is known as the Crocodile and 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.

Mr. Mnangagwa’s role as the likely successor to Mr. Mugabe has engendered some skepticism.

“He is now saying it is important to be part and parcel of what the people are saying when the people’s voices have been ignored so far,” said Okay Machisa, the executive director of ZimRights, a human rights group.

Lawmakers from the ruling ZANU-PF party, in discussions with the opposition, were struggling to find the right balance between two competing goals: removing Mr. Mugabe as quickly as possible while following the rule of law.

Politicians allied with Mr. Mnangagwa, who was voted the leader of ZANU-PF on Sunday, have vowed to impeach Mr. Mugabe in just a day or two by using the party’s overwhelming dominance in Parliament. But others warn that would amount to a kangaroo court.

Rushed proceedings could also taint a new government under Mr. Mnangagwa who, in his statement, promised a “new era” in which corruption was not tolerated.

“If we want to build a society that respects human rights and follows our Constitution, we need to make sure that all citizens, including the president, have the right to a fair trial,” said Rashid Mahiya, the executive director of Heal Zimbabwe, a human rights group. “Otherwise, this would be a big mistake for the precedent it sets.”

Many Zimbabweans interviewed in central Harare said that Mr. Mugabe did not deserve the right to reply in his defense.

“Mugabe has no right to challenge impeachment,” said Cyphas Ton’a, 53, a taxi driver. “He must just go and stay home. He wants to rule the country until when?”

But Allan Musvetu, 30, a ZANU-PF supporter, said that Mr. Mugabe should be allowed to defend himself because he was “a father figure” to the nation. Mr. Mugabe led Zimbabwe’s independence movement, and has been the country’s only president since it achieved nationhood in 1980.

“The voice of a child and a father are different because the father commands authority,” Mr. Musvetu said. “He must be given the right to reply because he is a father.”

In keeping with efforts to minimize the backlash against last week’s intervention, the military allowed Mr. Mugabe to try to convene a cabinet meeting Tuesday morning. In a surreal scene, just five ministers turned up as 17 others attended impeachment meetings.