Zimbabwean President Robert Mugabe turned 93 on Tuesday amid growing concerns over his frailty and failing health.

The ruling ZANU-PF party is to spend about $2.5 million on the birthday celebration, local media reports said.

Opposition parties condemned the events as a waste of money while "93 percent of Zimbabweans are wallowing in poverty caused by his incompetence and misrule", NewZimbabwe.com reported.

Celebrations for the world's oldest national ruler, who has vowed to remain in power, will be held on Saturday at Matobo National Park outside Bulawayo, Zimbabwe's second-largest city.

Thousands of officials and supporters of Mugabe's ZANU-PF party are expected to attend.

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Large game animals are often slaughtered for the occasion.

In previous years, Mugabe has reportedly been offered elephants, buffalo and impala for the feast.

'Call to resign must come from party'

Mugabe has ruled out retiring soon, saying that ZANU-PF officials believe there is no "acceptable" alternative.

"The call to step down must come from my party ... In such circumstances, I will step down," the state-owned Sunday Mail newspaper quoted Mugabe as saying in an interview aired late Monday.

"They want me to stand for elections ... If I feel that I can't do it any more, I will say so to my party so that they relieve me. But for now, I think I can't say so," he said. "The majority of the people feel that there is no replacement, a successor who to them is acceptable.

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Mugabe came to power when Zimbabwe won independence in 1980 and his rule has been criticised for repression of dissent, election rigging, and for causing the country's economic collapse.

Health fears

Several incidents in recent years have highlighted his advanced age - including a fall in February 2015 at Harare airport.

In September of the same year, he read a speech to parliament apparently unaware that he had delivered exactly the same address a month earlier.

Despite growing calls to step aside, his party has endorsed him as its candidate for general elections next year, and he remains widely respected as a liberation hero by other African leaders.

On Friday, his 51-year-old wife Grace claimed that Mugabe would be the voters' choice even after he dies.

She has promised to use a wheelchair to transport him to election rallies if needed.

Mugabe has avoided naming a successor, and his party is divided between factions hoping to succeed him.

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Grace was appointed head of the ruling party's women's wing in a surprise move that could make her a possible successor.

Another leading candidate is Mugabe's vice president, Emmerson Mnangagwa.

Last year, security forces brutally quelled a series of street protests in Harare, a rare public expression of opposition to Mugabe's regime.

According to Bloomberg News, Zimbabwe's economic output has halved since 2000 when many white-owned farms were seized by ZANU-PF supporters, leaving the key agricultural sector in ruins.