Zimbabwean President Robert Mugabe has said he would not "impose" his successor, as he celebrated his 93rd birthday.

He said that if the ruling party felt he should retire, it will hold an extraordinary congress to pick a new leader.

Mugabe, the world's oldest leader, has maintained a tight grip on power in the African country since its independence from Britain in 1980.

He is due to stand for re-election next year and says that the ruling party ZANU-PF has no viable alternative candidates for the presidency.

Mugabe was joined by thousands of supporters to celebrate his birthday at a school in Matobo outside the second largest city of Bulawayo.

"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 wife had previously said that his corpse could run for re-election next year.