Zimbabwe's President Robert Mugabe has marked his 93rd birthday with lavish celebrations, addressing his own mortality in a speech, but showing no signs of quitting politics.

Thousands of government supporters, some wearing clothing adorned with Mugabe's image in younger days, converged in Matabeleland for Saturday's birthday bash and show of strength for the ruling ZANU-PF party.

Also wearing a multi-coloured suit emblazoned with his own image and dark glasses, Mugabe, the world's oldest head of state, told the crowd he had no intention of stepping down.

"People who are busy forming their own groupings saying 'Mr Mugabe must go'. I ask myself where should I go?" Mugabe, who has been in power for 37 years, said in a speech that was broadcast on state radio and television.

Speaking for over an hour, Mugabe said he would not impose his successor and that if ZANU-PF party felt he should retire, it would hold an extraordinary congress to choose a new leader.

"Others are saying 'President, choose a successor before you retire'. Is that not imposition? Me imposing someone on the party? No, I don't want that," Mugabe said.

"This is an issue for the congress to choose. We can have an extraordinary congress if the president retires, but you said I should be your candidate in the next election."

His hands gripping the podium, Mugabe spoke slowly. He said that at times he felt alone, but that he has a "mission" and "mandate" as Zimbabwe 's leader and appealed for the ruling party to overcome its divisions.

"It's not always easy to predict that, although you are alive this year, you will be alive next year," he said.

"It does not matter how healthy you might feel. The decision that you continue to live and enjoy life is that of one personality we call the Almighty God."

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Guests at the birthday party enjoyed pieces of a cake weighing 93kg, according to party organiser Kudzai Chipanga.

The cake was shaped like the continent of Africa because "everyone in Africa loves him", said Chipanga.

ZANU-PF officials said hundreds of thousands of dollars were spent on the birthday bash at a school in Matobo, just outside Zimbabwe's second-largest city, Bulawayo.

But critics said the money should have been used for other priorities amid a deepening economic crisis .

Zimbabwe's economy is set to contract by 2.5 percent this year, according to the International Monetary Fund .

More than 80 percent of the workforce scrape a living in the informal sector, while the government has failed to pay civil servants' salaries on time since June.

Mugabe celebrated his actual birthday on February 21 with a smaller cake-cutting ceremony.