Harare: Zimbabwe's 90-year-old President Robert Mugabe fell down a staircase today as he walked off a podium after addressing supporters at Harare international airport.

He had just returned from Ethiopia where he took over the rotating chairmanship of the African Union.

Africa's oldest leader, who turns 91 later this month, had concluded his homecoming speech when he tumbled to his knees on a short flight of stairs in an incident witnessed by journalists and hundreds of supporters.

He was quickly helped up by aides and walked to a waiting car. Mugabe took over the post of African Union chairman on Friday, replacing Mauritania's President Mohamed Ould Abdel Aziz.

In his speech, Mugabe said he felt honoured that Zimbabwe was elected chair of the AU, pledging to use his position to push for development in the continent.

"The AU must be the engine of development in Africa. We should be the engine of development in Africa," Mugabe said. "We want real progress to be achieved to develop all our countries."