JOHANNESBURG -- FW de Klerk, the last president of South Africa during apartheid, hailed Nelson Mandela as one of the greatest figures of the last century in a 90th birthday tribute on Thursday to his successor.

De Klerk, who stood down after Mandela won the first multi-racial elections 14 years ago, said his co-winner of the 1993 Nobel peace price was a born leader with the "humility and the grace of a true natural aristocrat."

"He is the most famous South African who has ever lived and is universally regarded as one of the greatest figures of the 20th century," said de Klerk in a statement.

The former National party leader acknowledged his working relationship with Mandela went through some rough patches both in negotiations leading up to the end of whites-only rule in 1994 and in a two-year stint as deputy president.

"He was a hard, sometimes remorseless, negotiating partner and our relationship was often severely strained," he said.

"Nevertheless, whenever the situation demanded it ... we were able to overcome our differences and take concerted action to defuse the crises as they arose.

"After his inauguration, Nelson Mandela used his personal charm to promote reconciliation and to mould our widely diverse communities into an emerging multicultural nation. This, I believe, will be seen as his greatest legacy."

Mandela, who turns 90 on Friday, served one term as South African president before standing down and being succeeded by the current head of state Thabo Mbeki in 1999.