Zimbabwean president Robert Mugabe has celebrated his 93rd birthday surrounded by thousands of supporters in a defiant show of strength after nearly four decades in power.

Wearing dark glasses and a multi-coloured jacket bearing an image of himself in younger days, Mugabe hosted a birthday celebration in Matabeleland attended by government supporters and delegates from ruling parties in Botswana, Namibia, Angola and Zambia.

In a speech, Mugabe described his life as a "long, long journey," saying it was difficult to predict how long it would last.

"The decision that you continue to live and to enjoy life is that of the one personality we all call the almighty God," he said.

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

Zimbabwe's challenges include drought and a strike by doctors over working conditions that has forced army and police doctors to deploy in public hospitals.

The government has endured other crises, rejecting decades of opposition and Western allegations about human rights violations, voting irregularities and economic mismanagement.

Mugabe, who turned 93 on Tuesday, has also been serenaded at a palace cake-cutting by singers who wished him "many more" birthdays.

Air Zimbabwe, the cash-strapped national carrier, and other entities took out birthday notices in pro-government media.