Zimbabwe's President Robert Mugabe sits for formal photographs after a student graduation ceremony at Zimbabwe Open University. Picture: Ben Curtis/AP

Harare - Zimbabwe's President Robert Mugabe appeared in public on Friday for the first time since the army took charge, as the ruling party made plans to force him to step down after more than three decades in power.

The president, who is 93, opened a graduation ceremony at Zimbabwe Open University in Harare. He wore blue and yellow academic robes and a mortar board hat and appeared to fall asleep in his chair as his eyes closed and his head lolled.

Mugabe led the country's liberation struggle and has dominated its politics since independence in 1980 but this week's army takeover signals the collapse of his authority despite his insistence he remains in charge. A senior member of the Zanu PF ruling party said it wanted him gone.

"If he becomes stubborn, we will arrange for him to be fired on Sunday," the source said. "When that is done, it's impeachment on Tuesday."

In contrast, the military said in a statement on national television it was "engaging" with Mugabe. It referred to him as Commander in Chief and said it would announce an outcome as soon as possible.

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The ruling Zanu PF party has called for a mass meeting in the capital on Saturday to show its support for the War Veterans group in their bid to remove Mugabe.

Mugabe is revered as an elder statesman and independence leader but he is also viewed by many in Africa as a president who crippled his country by remaining in power too long. He calls himself the grand old man of African politics.

The army appears to want him to go quietly and allow a transition to Emmerson Mnangagwa, whose sacking last week as vice president triggered the takeover.

A goal of the generals is to prevent Mugabe handing power to his wife, Grace, who appeared on the cusp of power after Mnangagwa was pushed out.

"NO GOING BACK"

Zimbabwe's official newspaper, the Herald, ran photographs late on Thursday showing Mugabe grinning and shaking hands with military chief General Constantino Chiwenga, who seized power this week.

The images stunned Zimbabweans who said it meant Mugabe was managing to hold out against Chiwenga's coup. Some political sources said he was trying to delay his departure until elections scheduled for next year.

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The Zanu PF source said that was not the case. Anxious to avoid a protracted stalemate, party leaders were drawing up plans to dismiss Mugabe at the weekend if he refused to quit, the source said.

"There is no going back," the source told Reuters. "It's like a match delayed by heavy rain, with the home side leading 90-0 in the 89th minute."

The army is camped on his doorstep. Grace Mugabe is under house arrest and her key political allies are in military custody. The police, once a bastion of support, have showed no signs of resistance.

Furthermore, he has little popular backing in the capital, a stronghold of support for opposition parties that have tapped into the anger and frustration at his handling of the economy, which collapsed after the seizure of white-owned farms in 2000.

Unemployment is now running at nearly 90 percent and chronic shortages of hard currency have triggered hyperinflation, with the prices of imports rising as much as 50 percent a month.

Mugabe has won a series of elections but his critics in Africa and the West say his handling of the economy has been disastrous and he has used violence to maintain power. Botswana's President Ian Khama told him to resign.

"I don't think anyone should be President for that amount of time. We are Presidents. We are not monarchs. It's just common sense," Khama said.

Speaking at a Southern African Development Community meeting in Botswana's capital Gaborone, South Africa's President Jacob Zuma struck a different chord.

"We note with great concern the unfolding political development in Zimbabwe and we hope that they will not lead to unconstitutional change of government," Zuma said.

"VERY STUBBORN"

Dumiso Dabengwa, a Zimbabwean liberation war veteran and KGB-trained former intelligence chief, said Mugabe's fate was sealed but old age was causing him to dig in his heels.

"At his age, everybody becomes very stubborn and he would be no exception," he told reporters in Johannesburg.

"He certainly will not do it easily, but I think the people will show him that he's no longer wanted."

U.S. Secretary of State Rex Tillerson on Friday urged "a quick return to civilian rule" in Zimbabwe.

"Zimbabwe has an opportunity to set itself on a new path, one that must include democratic elections and respect for human rights," Tillerson told the foreign ministers from the African continent ahead of a meeting in Washington.

The United States, a longtime Mugabe critic, is seeking "a new era", the State Department's top official for Africa said, an implicit call for Mugabe to quit.

Read more: Botswana's Khama tells Mugabe to go

In an interview with Reuters, acting US Assistant Secretary of State for African Affairs Donald Yamamoto appeared to dismiss the idea of keeping Mugabe in an interim or ceremonial role.

"It's a transition to a new era for Zimbabwe, that's really what we're hoping for," Yamamoto said.

For its part, China's Foreign Ministry called for a peaceful resolution in Zimbabwe under a legal framework.