Zimbabwean President Robert Mugabe said on Saturday he would not impose his successor and that if the ruling ZANU-PF party felt he should retire, it would hold an extraordinary congress to choose a new leader.

The world's oldest leader, who turned 93 this week, has maintained a tight grip on power in Zimbabwe since independence from Britain in 1980. He is due to stand for re-election next year and says ZANU-PF has no viable alternative candidates.

Mugabe was joined by thousands of supporters to celebrate his birthday at a school in Matobo just outside the second biggest city Bulawayo, and a short distance from the grave of colonialist Cecil John Rhodes.

Rhodes was an imperialist, businessman and politician who played a dominant role in Southern Africa in the late 19th century, driving the annexation of vast swathes of land.

"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."

Matobo is one of the areas that suffered heavy casualties during the 1980s crackdown by an elite North Korean-trained brigade against rebels loyal to Joshua Nkomo, Mugabe's then rival. Rights groups say 20,000 civilians died during the so-called Gukurahundi offensive.

Critics say Mugabe's policies, like the seizure of white-owned farms for blacks and his black economic empowerment drive have ruined the once promising country.

Mugabe, a pariah in the West, said he agreed with U.S. President Donald Trump's "America for Americans" approach and criticised Zimbabweans seeking jobs in America.

Mugabe attributed his long life to what he said was God's mission to "fulfil the needs and demands" of Zimbabweans.

"I thank the Lord and say I accept the mission my Lord," said Mugabe, who wore a black cowboy hat with national colours and a print shirt designed by his wife Grace.

Mugabe said he only had one sister left and missed his siblings, adding that "when I look back I say aah, oh Lord, why were these taken before me and why have I remained so long, alone and alive. I cannot answer that."

A local bakery donated a 96 kg cake designed in the shape of the Zimbabwean map, while party officials jostled to give Mugabe gifts ranging from sculptures to cattle.