By MacDonald Dzirutwe

HARARE (Reuters) - Zimbabwean President Robert Mugabe faces the start of impeachment proceedings on Tuesday that could see him ousted within the week, against the backdrop of a military takeover dubbed "Operation Restore Legacy".

The ruling ZANU-PF party plans to bring the impeachment motion in parliament, after a Monday noon deadline expired for the besieged 93-year-old leader to resign and bring the curtain down on nearly four decades in power.

Impeachment would be an ignominious end to the career of the "Grand Old Man" of African politics, once lauded as an anti-colonial hero and the only leader Zimbabwe has known since it gained independence from Britain in 1980.

Mugabe has so far shown no signs of stepping down and has called for the weekly cabinet meeting to take place as usual on Tuesday. It would be the first time ministers sit down with him since the military took power on Wednesday.

In the draft impeachment motion, ZANU-PF - which expelled Mugabe from the party on Sunday - accused him of being a "source of instability", flouting the rule of law and presiding over an "unprecedented economic tailspin" in the last 15 years.

It also said he had abused his constitutional mandate to favour his unpopular wife Grace, 52, whose tilt at power triggered the backlash from the army that brought tanks onto the streets of the capital last week.

The military operation was launched after Robert Mugabe sacked former vice president Emmerson Mnangagwa, in a move meant to boost Grace's chances of succeeding her husband.

Zimbabwe's top general said on Monday talks were planned between Mugabe and Mnangagwa, who was expected back in the country soon.

General Constantino Chiwenga also revealed that the army's intervention was codenamed "Operation Restore Legacy" and was progressing well.

FALL OF GRACE

It has been marked by unexpected twists and turns.

On Saturday, hundreds of thousands took to the streets of Harare to celebrate the impending downfall of Mugabe, accused by critics of retaining power through terror and election-rigging and of running a once-vibrant economy into the ground.

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They expected him to resign within hours. Instead Mugabe dashed their hopes with a bizarre and rambling televised address on Sunday night in which he made no mention of his own fate.

Since last week, Mugabe has been confined to his lavish "Blue Roof" residence in Harare, apart from two trips to State House to meet the generals and one to a university graduation ceremony at which he appeared to fall asleep.

Grace, known as "Gucci Grace" for her alleged fondness for extravagant shopping sprees, and at least two senior members of her "G40" political faction are believed to be holed up in the same compound.

Her stark reversal of fortune was underscored on Monday when the state-run Herald newspaper - which in August proclaimed her "A loving mother of the nation" - ran a piece headlined "Youth League slams 'uncultured' First Lady."

"Grace Mugabe lacked grooming and true motherhood as shown by her foul language," the paper quoted the ZANU-PF's youth wing as saying.

(Writing by Ed Stoddard; editing by Andrew Roche)