Nelson Mandela says there is a tragic failure of leadership in Zimbabwe.

It is the first time the former South African president has spoken publicly about Zimbabwe's political crisis.

Mandela spoke at a dinner in London on Wednesday. He told the gathering there is a "tragic failure of leadership" in Zimbabwe.

Zimbabwean President Robert Mugabe has been under pressure to call off Friday's presidential runoff. His opponent dropped out of the race saying he could not ask his supporters to risk their lives.