Zimbabwe's first lady says she plans to get a "special" wheelchair for her 91-year-old husband so can continue to rule.

Speaking at a political rally on Saturday, Grace Mugabe told supporters that she would get a wheelchair for her husband, President Robert Mugabe, to use.

In the televised speech, Mugabe, 50, vowed to push the wheelchair herself, taking a swipe at critics who say her husband is too old to govern.

Mugabe told supporters she would ensure her husband remains in power as long as he can still speak.

The Zimbabwean president's critics have questioned his ability to rule after he read the wrong speech in parliament. Several stumbles at public events have made headlines.

Robert Mugabe has led Zimbabwe since independence in 1980.