Just two weeks ahead of Zimbabwe's run-off election, President Thabo Mbeki of South Africa met with President Robert Mugabe in what appeared to be a last-minute effort to avoid any tampering to the ballot in an election already marred by violence and intimidation.

Mbeki is leading regional efforts to resolve the political turmoil Zimbabwe has been in, but critics say he and other African leaders have not taken a strong enough stand against Mugabe.

Government sources said Mbeki, the official mediator for Zimbabwe, arrived in the capital Harare just before 1pm local time (1100 GMT) and went straight into a meeting with the South African ambassador.

Some African leaders are growing impatient with the Mugabe government, however. Kenyan Prime Minister Raila Odinga said this week that Zimbabwe's first presidential election in March was rigged. He urged Mr. Mugabe to step down from office, after ruling Zimbabwe since 1980.

Former U.N. Secretary-General Kofi Annan wrote in Britain's Financial Times Wednesday that the winner of an unfair vote will not have the legitimacy to govern Zimbabwe.

Mbeki has been appointed to mediate Zimbabwe's crisis by the 14-nation Southern African Development Community (SADC), and has often come under fire for his policy of quiet diplomacy towards the neighboring country.

Violence has increased in Zimbabwe ahead of the June 27 run-off election, in which opposition Movement for Democratic Change (MDC) leader Morgan Tsvangirai is seeking to topple Mugabe who is battling to keep his 28-year hold on power in a country suffering economic collapse.