"When asked who they thought was the greatest leader of all time, 71.6 percent voted for Nelson Mandela, seven percent for Barack Obama, 6.7 percent for Martin Luther King and 3.8 percent for Gandhi," researchers said.

Pondering Panda conducted the survey among 4500 South Africans aged 18 to 34 via cellphone last week.

Mandela celebrates his 94th birthday on Wednesday.

Asked who was the best South African president since democracy, 84 percent of respondents answered Mandela, seven percent said Thabo Mbeki and four percent voted for President Jacob Zuma.

Five percent of respondents were undecided.

"Although the overwhelming majority felt that Mandela had been the best president since democracy, they also felt that the ANC had let him down," Pondering Panda said.

Sixty percent of respondents said the African National Congress had not stayed true to Mandela's dream for South Africa's future, while 35 percent said the party had stayed true. The rest were undecided.

"There were strong demographic differences in responses, with black respondents being most sympathetic to the ANC's efforts -- 45 percent feeling that the ANC was indeed doing what Mandela would have wanted."

However, 80 percent of Indian, 76 percent of white, and 73 percent of coloured South Africans felt that the ANC had let Mandela down.

"Women were also more negative than men, with 67 percent feeling that the ANC had not lived up to his dream."

Respondents in Limpopo, at 84 percent, felt the strongest about Mandela being the greatest leader of all time.

This was closely followed by those in the Northern Cape at 83 percent.

Consumer insights company Pondering Panda is a member of the World of Avatar group, which also owns Mxit, a mobile social network.