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Nelson Mandela's foundation has vowed to press ahead with upcoming 95th birthday celebrations, as the global peace icon underwent a 12th day in hospital Wednesday for a recurring lung infection.

On his July 18 birthday, various charity organisations and businesses will paint schools, hand out food parcels and books, and run a 67 kilometre (41 miles) relay marathon in the spirit of Mandela's 67 years of activism and public work.

Mandela spent 27 years in jail for his opposition to white minority rule, and walked free to become the country's first black president after all-race elections in 1994.

"We see this as an opportunity for people to rise above Madiba's hospitalisation," said Sello Hatang, the foundation's CEO, referring to Mandela by his clan name.

The Nobel peace laureate, who devoted his life to promote education and AIDS activism, wanted people to carry on his good work, his great-grandson Mbuso Mandela told AFP.

"His idea for me was just to continue the legacy which he put forth to myself and to the rest of the nation, which is humility and education," said the 22-year-old.

Local and global celebrities have also endorsed a call by the United Nations, which declared the former president's birthday International Mandela Day, on people to do 67 minutes of good deeds.

"I too will also be giving my 67 minutes to make the world a better place, one small step at a time," said British billionaire Richard Branson in a video clip played at this year's International Mandela Day launch in Johannesburg.

Mandela's birthday celebrations will be all the more poignant as the icon battles a persistent pulmonary condition that has plagued him for years.

He was hospitalised on June 8 in the capital Pretoria with a lung infection, and the South African presidency has described his condition as serious but improving.

But the United Nations insisted on his living legacy, instead of the ailing man.

"People are focusing more on his passing away rather than his recovery," said UN country representative Agostinho Zacarias.

"We at the UN would like to focus on his recovery and a quick recovery, and wish him well."

Former US president Bill Clinton said following Mandela's example starts with small actions.

"In doing so we can inspire more and more people to live as Mandela has: with the belief that tomorrow does not have to be like yesterday," he said in pre-recorded message.

Another great-grandson, Luvuyo Mandela, 27, said his world-renowned relative would have been proud to see people echo his goodwill in their deeds.

"How would he respond? With his world-famous smile," said the younger Mandela.