Prime Minister Gordon Brown has called Nelson Mandela the "greatest and most courageous leader of our generation" as he welcomed him to Downing Street.
Mr Brown paid tribute to Mr Mandela's courage
The former South African president is in London to witness a 9ft-high (2.7m) bronze statue of himself being unveiled in Parliament Square.
The pair met for private talks ahead of the unveiling on Wednesday.
A Downing Street aide said the pair had known each other a long time and the talks were a chance to "catch up".
Mr Mandela said he was proud to be inside Number 10, joking: "My wife and I are happy to be here because, as you know, this was one of our rulers, but we overthrew them.
"We are on an equal basis now."
Mr Brown added: "I'm proud to welcome to Downing Street the most inspiring, the greatest and most courageous leader of our generation.
"It's such a privilege to have Nelson Mandela and his wife, Graca Machel, here today and I'm looking forward to our discussions."
He told Mr Mandela: "I'm looking forward also to the statue being inaugurated in your honour tomorrow in London."
The prime minister and Mr Mandela will join London Mayor Ken Livingstone at the ceremony outside the Houses of Parliament.
It follows a five-year row over the location of the statue - Mr Livingstone had wanted it to be put up in Trafalgar Square.
But he said Parliament Square - home to statues of Sir Winston Churchill, Benjamin Disraeli and South African statesman Field Marshal Smuts - reflected Mr Mandela's significance as a world statesman.