The Latest on Zimbabwe's political turmoil (all times local):

6 p.m.
Zimbabwe's President Robert Mugabe says he is resigning immediately and voluntarily in order to have a "smooth transfer of power" after 37 years in charge.
The letter was read out in a cheering, dancing Parliament, which had been pursuing impeachment of the 93-year-old Mugabe, the world's oldest head of state.
The resignation comes at the end of a week of extraordinary events that began with the military moving in last week, angered by Mugabe's firing of his longtime deputy and the positioning of the unpopular first lady to succeed him.
Impeachment allegations against Mugabe included that he "allowed his wife to usurp constitutional power" and that he is "of advanced age" and too incapacitated to rule.
Mugabe also was accused of allowing unpopular first lady Grace Mugabe to threaten to kill the recently fired Vice President Emmerson Mnangagwa and other officials.
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5:55 p.m.
Zimbabwe's Parliament has erupted in cheers as the speaker announces the resignation of President Robert Mugabe.
The speaker stopped impeachment proceedings to say they had received a letter from Mugabe with the resignation "with immediate effect."
It is an extraordinary end for the world's oldest head of state after 37 years in power.
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5:25 p.m.
An expert on Zimbabwean law says impeachment is a process that requires a vote, a committee investigation and a second vote.
Derek Matyszak, senior researcher for the Institute for Security Studies, says the first step is for both houses of Parliament to pass the impeachment motion by a 50 percent majority. Then a joint committee is formed to investigate allegations and determine if there is adequate evidence that Mugabe should be impeached.
If the committee recommends impeachment, both houses must pass the impeachment by a two-thirds majority, which is at least 233 seats of the 347-seat total.
Matyszak says that "the moment they vote to accept the report and impeach Mugabe, he loses office. The constitution is clear about that. Mugabe can appeal to the judiciary but he would be out of office."
It is not clear how long the process will take or how extensive the investigation would be, including with possible testimony from Mugabe.
Matyszak says that "I think it will be fast-tracked, but they want to take enough time to give the proceedings an air of propriety. They want this to look like a legitimate proceeding. That could be done so the final vote is on Wednesday or Thursday."
Parliament is currently hearing allegations and has not yet formed a committee.
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5:15 p.m.
Zimbabwe's Parliament members are cheering as they listen to allegations against President Robert Mugabe as they enter the next stage of the impeachment process.
Parliament is now forming a committee to investigate the allegations against Mugabe, including that he "allowed his wife to usurp constitutional power" and that he is "of advanced age."
Mugabe also is accused of allowing unpopular first lady Grace Mugabe to threaten to kill the recently fired Vice President Emmerson Mnangagwa and other officials.
In addition, "we have seen the president sleeping in Cabinet and international meetings to the horror, shame and consternation of Zimbabweans."
The impeachment motion was introduced by the ruling party and seconded by the opposition MDC. It is not clear how long the process will take.
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4:50 p.m.
Bells have rung to summon Zimbabwe's Parliament members to gather for the next stage of the impeachment process for President Robert Mugabe.
Parliament is now forming a committee to investigate the allegations against Mugabe, including that he "allowed his wife to usurp constitutional power" and that he is "of advanced age" and no longer has the physical capacity to run the government.
The impeachment motion was introduced by the ruling party and seconded by the opposition MDC. It is not clear how long the process will take, though lawmakers have said Mugabe could be voted out as early as Wednesday or Thursday.
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3:45 p.m.
South Africa's state-run broadcaster reports that the presidents of South Africa and Angola will travel to Zimbabwe on Wednesday to meet with "stakeholders" in the political crisis, including President Robert Mugabe and the military.
Zimbabwe's Parliament has begun impeachment proceedings against Mugabe, who is accused of allowing his wife to "usurp" power and of being too old to rule.
It is not clear how long impeachment would take, though the ruling party has said it could vote Mugabe out as early as Wednesday.
South Africa, Angola and two other regional countries held a summit on the crisis Tuesday in Angola.
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3:30 p.m.
Zimbabwe's ruling party has made a motion to impeach President Robert Mugabe and opposition party MDC has seconded it.
Now lawmakers from both houses of Parliament are relocating to a larger venue.
Mugabe is accused of allowing his wife to "usurp" power and of being too old to rule. He has been in charge since the end of white minority rule in 1980, but the military moved in last week after he fired his deputy and appeared to position his unpopular wife to succeed him.
It is not clear how long impeachment would take, though the ruling party has said it could vote Mugabe out as early as Wednesday.
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3:10 p.m.
Zimbabwe opposition leader Morgan Tsvangirai is telling a crowd outside Parliament that a "democratic Zimbabwe cannot be built by another un-democratic process."
Tsvangirai says the culture of the ruling party "must end" and everyone must put their heads together and work toward free and fair elections.
"Now the question is, how do we end Mugabe," says the opposition leader, who shared power with Mugabe as prime minister for a number of years.
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2:45 p.m.
Parliament has opened in Zimbabwe as the ruling party seeks to impeach President Robert Mugabe after nearly four decades in power.
Mugabe is accused of allowing his wife to "usurp" power and of being too old to rule.
Mugabe has been in charge since the end of white minority rule in 1980, but the military moved in last week after he fired his deputy and appeared to position his unpopular wife to succeed him.
Zimbabweans are rallying outside Parliament. It is not clear how long impeachment would take, though the ruling party has said it could vote Mugabe out as early as Wednesday.
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2:20 p.m.
Zimbabweans are rallying outside Parliament as the ruling party is poised to begin impeachment proceedings against President Robert Mugabe, who is accused of allowing his wife to "usurp" power and of being too old to rule.
"We are here because we want to be part of this very important occasion in the history of this country," says Harare resident Samuel Wadzai.
Lawmakers have begun arriving at Parliament.
Mugabe has been in power since the end of white minority rule in 1980, but the military moved in last week after he fired his deputy and appeared to position his unpopular wife to succeed him.
The pastor who last year led the country's largest anti-government protests in a decade, Evan Mawarire, tells the crowd that "our country was a shame and we were embarrassed by it. But today we declare that we love Zimbabwe."
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2 p.m.
Botswana's government has posted online what it calls an open letter from President Ian Khama urging Zimbabwe President Robert Mugabe to step down.
Khama has openly criticized his neighbor in the past. His letter posted on social media comes as Zimbabwe's ruling party is poised to begin impeachment proceedings against Mugabe.
The letter asks the world's oldest head of state to "be sensitive to the wishes of the people of Zimbabwe and to do the honourable thing by voluntarily relinquishing power."
Khama adds that Zimbabwe's people have been "subjected to untold suffering" under Mugabe, who has ruled for 37 years while the once-prosperous southern African nation's economy collapsed.
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12:50 p.m.
A Zimbabwean ruling party senior official says Cabinet ministers have snubbed a call by President Robert Mugabe to attend a meeting.
Lovemore Matuke says "all" ministers heeded a party directive to skip the Cabinet meeting and instead attend a party caucus to discuss impeaching Mugabe.
The party's Central Committee has voted to strip Mugabe of his party leadership post amid nationwide calls for the 93-year-old leader to resign. Mugabe, however, says he plans to preside over a ruling party congress next month.
The ruling party is poised to begin impeachment proceedings against Mugabe on Tuesday.
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10:25 a.m.
Four regional countries are meeting on Zimbabwe's political crisis.
South Africa's president is joining the Angola-hosted summit of the Southern African Development Community, along with the leaders of Zambia and Tanzania.
A committee of the regional bloc has recommended a full summit of all 16 members to discuss Zimbabwe, where longtime President Robert Mugabe faces impeachment after his firing of his deputy and positioning of the unpopular first lady to succeed him.
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9:45 a.m.
The Zimbabwe vice president whose firing kicked off the country's political crisis says President Robert should heed the "clarion call" and resign immediately: "The people of Zimbabwe have clearly spoken."
The new statement from Emmerson Mnangagwa makes clear that he remains outside Zimbabwe after fleeing and won't return until his security is guaranteed.
Mnangagwa confirms that Mugabe has invited him to return "for a discussion" on the recent events. But "given the events that followed my dismissal I cannot trust my life in President Mugabe's hands."
He says his security was withdrawn upon his firing and he was informed that "plans were underfoot to eliminate me once arrested."
Mnangagwa says he is aware of the impeachment proceedings that start Tuesday against Mugabe and "I will not stand in the way of the people and my party."
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9:20 a.m.
A recently fired Zimbabwean vice president and likely successor to President Robert Mugabe says the 93-year-old leader should resign immediately.
Emmerson Mnangagwa says in a statement Tuesday that he is not in Zimbabwe and that he would not return to the country until he is "satisfied of my personal security."
Mugabe fired Mnangagwa earlier this month, but Zimbabwe's ruling party is demanding that Mugabe resign and wants the former vice president to replace him.
The ruling party is poised to begin impeachment proceedings against Mugabe on Tuesday as Parliament resumes, and it has instructed government ministers to boycott a Cabinet meeting Mugabe has called for Tuesday morning.
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9 a.m.
Zimbabwe's ruling party is set to begin impeachment proceedings against longtime President Robert Mugabe, while a party official says government ministers have been instructed to boycott a Cabinet meeting called by the president.
Ruling party chief whip Lovemore Matuke tells The Associated Press minutes before the Cabinet meeting is expected to start that ministers have been told to instead attend a meeting at party headquarters to work on the impeachment. Parliament resumes Tuesday.
Mugabe's chief secretary on Monday summoned ministers to the Cabinet meeting at State House, the president's official residence.
Mugabe is finding himself increasingly isolated.
The military on Monday night said the vice president he recently fired, sparking the political turmoil, will return to Zimbabwe "shortly" and has made contact with Mugabe.

AP