Zimbabwean President Robert Mugabe has said his ruling ZANU-PF party and the people of Zimbabwe see no viable successor to him for next year's general elections.

Mugabe has been in power since 1980 and in December his party confirmed him as its candidate for the next presidential election expected in mid-2018, when he will be 94.

The world's oldest national leader, Mugabe turns 93 on Tuesday, with a celebration party planned for next Saturday.

"They want me to stand for elections, they want me to stand for elections everywhere in the party," Mugabe said in comments to state media.

"The majority of the people feel that there is no replacement, successor who to them is acceptable, as acceptable as I am."

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

On Friday, his wife Grace Mugabe said the 92-year-old would be the voters' choice even after he dies.

"One day when God decides that Mugabe dies, we will have his corpse appear as a candidate on the ballot paper," Grace, seen as a possible successor to her ailing husband, told a party rally in Buhera, southeast of the capital Harare.

"You will see people voting for Mugabe as a corpse. I am seriously telling you, just to show people how people love their president."

Grace, who was appointed leader of ZANU-PF's women's wing in a surprise move two years ago, is well known for her fiery speeches and verbal attacks on her husband's opponents.

In 2015, she led a campaign which led to the ousting of deputy president Joice Mujuru, accusing Mujuru of fanning factionalism, plotting to topple Mugabe and corruption and bribery.

Grace has previously pledged to push Mugabe in his wheelchair to election rallies if needed.