HARARE (AFP) - Zimbabwean President Robert Mugabe, the world's oldest national ruler, turned 93 on Tuesday (Feb 21), using a long and occasionally rambling interview to vow to remain in power despite growing signs of frailty.

He celebrated the day at a private event in Harare with a cake and presents from staff as supporters and ruling Zanu-PF party officials filled state media with gushing messages of goodwill and congratulations.

During an hour-long, pre-recorded television interview broadcast late on Monday, Mugabe appeared to grow increasingly tired, pausing at length between sentences and speaking with his eyes barely open.

"The call to step down must come from my party... In such circumstances, I will step down," he said.

"They want me to stand for elections... If I feel that I can't do it any more, I will say so to my party so that they relieve me. But for now, I think I can't say so.

"The majority of the people feel that there is no replacement - a successor who to them is acceptable."

Mugabe, who has ridiculed regular reports that he is close to death, spoke about creating jobs in Zimbabwe's wrecked economy, the country's extreme cash shortage and his much-criticised wife.

Sitting in State House, his official residence in Harare, he appeared lucid at some points while at other times he drawled and lost track of his thoughts.

The Zimbabwean opposition used the birthday to say Mugabe was an outdated leader, incapable of solving the country's problems.

The President's main birthday celebrations will be held on Saturday at Matobo National Park outside Bulawayo, where thousands of officials and Zanu-PF faithful are expected to gather.

Large game animals are often slaughtered for the occasion. In previous years, Mugabe has reportedly been offered elephants, buffalo and impala for the feast.

The veteran leader, who came to power when Zimbabwe won independence in 1980, is accused of holding onto office through ruthless repression of dissent and election-rigging, while overseeing an economic collapse.

The state-owned Herald newspaper published a 24-page supplement of congratulatory messages from government departments and regime loyalists, while state television and radio broadcast tributes and songs of praise.

"As we celebrate his 93rd birthday today, we do so in the knowledge and comfort that our country is in very good and capable hands," the Herald said in its editorial.

The defence ministry message read: "Your wise visionary leadership continues to provide us with clear direction and greater resolve to defend our country."

Several incidents in recent years have highlighted Mugabe's advanced age - including a public fall in 2015 at Harare airport.

In September of the same year, he read a speech to parliament apparently unaware that he had delivered the same address a month earlier.

His party has endorsed Mugabe as its candidate for general elections next year, and he remains widely respected as a liberation hero by other African leaders.

On Friday, his wife, Grace, 51, claimed that Mugabe would be the voters' choice even after he dies.

Grace, who has a reputation for extravagance and fierce verbal attacks on rivals, has also said she would use a wheelchair to transport him to election rallies if needed.

She was appointed head of the Zanu-PF women's wing in a surprise move that could make her a possible successor to Mugabe.

"She is very acceptable. Very much accepted by the people," Mugabe said in the interview, without giving further details.

Another leading candidate is Mugabe's vice-president, Emmerson Mnangagwa.

A spokesman for the Movement for Democratic Change opposition party wrote a stinging opinion piece in the privately-owned NewsDay paper.

"Today's problems need today's people. Yet you do not even belong to yesterday's generation. At 93, you certainly belong to yesterday but one," wrote Luke Tamborinyoka.

Last year, security forces quelled a series of street protests in Harare that were a rare public expression of opposition to Mugabe's regime.

According to Bloomberg News, Zimbabwe's economic output has halved since 2000 when many white-owned farms were seized, leaving the key agricultural sector in ruins.