Harare, Zimbabwe - Pressure is mounting on Robert Mugabe to step down as Zimbabwe's president, as efforts to force the veteran leader to resign after nearly four decades in office gain traction.

Zimbabwe has been in political turmoil since the early hours of Wednesday, when the country's armed forces seized power and placed Mugabe under house arrest.

The embattled 93-year-old leader so far appears resistant to demands to step down. However, he is increasingly running out of options as even some of his closest allies, including top officials within his ruling ZANU-PF party, look determined to remove him from power.

Issuing a "stark warning", Zimbabwe's influential war veterans on Friday said Mugabe, the patron of their 35,000-strong association, should not be allowed to stay any longer in power.

"If he doesn't leave, we are going to settle the score," Chris Mutsvangwa, the war veterans' leader, told a press conference in Zimbabwe's capital, Harare.

"There is no going back about Mugabe - he must leave," added Mutsvangwa.

Mass demonstration

Adding to the pressure, the independence war fighters' association called for a mass rally in Harare on Saturday, urging residents to march to the State House in large numbers to demand Mugabe's resignation.

Thousands of people across the political spectrum are expected to take to the streets, including supporters of ZANU-PF and members of the opposition.

Evan Mawarire, leader of last year's anti-Mugabe #ThisFlag protests, also urged people to participate in the rally.

Tomorrow Zimbabweans stand together for a #FreshStart https://t.co/pFBnCumj6r — #ThisFlag E Mawarire (@PastorEvanLive) November 17, 2017

Earlier on Friday, Mugabe appeared in public for the first time since the army's takeover on Wednesday.

Dressed in a blue academic gown and hat, Mugabe attended a graduation ceremony at the Zimbabwe Open University in Harare where he was welcomed by a cheering crowd of supporters.

He entered the ceremony venue surrounded by security personnel and announced the opening of the event after joining the crowd in singing the national anthem.

Provincial branches step in

However, his political isolation gathered pace later on Friday as all 10 of ZANU-PF's provincial structures passed a motion of no-confidence and called on him to step down as first secretary.

In a rare show of defiance, the provincial branches' move was carried by Zimbabwe's state broadcaster ZBC.

Meanwhile, a private radio station owned by one of Mugabe's aides, Supa Mandiwanzira, the minister of information, communication, technology and courier services, on Friday broadcast messages calling on citizens to take to the streets on Saturday.

Following the votes by ZANU-PF's branches, the party's Central Committee is now expected to hold an emergency meeting on Sunday to pass a resolution of no-confidence in Mugabe's 40-year leadership of the party.

Such a move would be unprecedented as Mugabe has never faced a no-confidence vote or a similar collective leadership challenge since his election as ZANU-PF's leader in 1977 during the liberation struggle.

Mugabe has been leading the country since independence in 1980.

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If the proposal is approved, it will then be put on the agenda for the party's upcoming extraordinary conference in December.

In their list of resolutions, the provinces also demanded the reversal of former Vice President Emmerson Mnangagwa's expulsion from the party.

They also removed their endorsements of First Lady Grace Mugabe and demanded she step down from her post as first secretary of the party's Women's League.

The military demands seek to reinstate Mnangagwa as first vice president and then as leader of a transitional administration.

'His time is up'

Along with the march to State House in Harare, a demonstration is also expected to take place at Davis Hall, ZANU-PF's headquarters in Bulawayo, Zimbabwe's second city.

Temba Mliswa, an expelled ZANU-PF member and independent parliamentarian, said if Mugabe refused to step down, a motion to pass a no-confidence vote would be put before parliament at its next sitting on Tuesday.

"He is an old man, he is sick and he is in a state of shock so he needs to digest the news because he never imagined this day would come," Mliswa told Al Jazeera.

"He can no longer resist, he should know his time is up."

Mediation talks to resolve the political crisis are currently taking place in Zimbabwe and Botswana where the Southern African Development Community is expected to hold an extraordinary meeting.

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