Nelson Mandela spent his 95th birthday in hospital Thursday but his health was "steadily improving", the South African presidency said, as people around the world honoured his legacy with charitable acts.

With a wave of good deeds planned to mark Nelson Mandela Day, South Africans awoke to word that their national hero was getting better six fraught weeks after he was admitted to hospital with a recurring lung infection.

"Madiba remains in hospital in Pretoria but his doctors have confirmed that his health is steadily improving," the presidency said in a statement, using Mandela's clan name.

Ndileka Mandela said her grandfather was now more responsive. "He's using his eyes, nodding," she told AFP.

The news prompted joyful prayers of thanks outside the hospital which, for 41 days, has been the focal point of a national vigil.

"He is here with us today us. We didn't know that he will live to see this day," said Agnes Shilowane, a local university student. "Tata (father) Mandela has once again proved that he is a fighter."

Nearby, school children read poems and left cards as nurses sang happy birthday to the former political prisoner who went on to become South Africa's first black president.

Birthday greetings and messages of support poured in from around the world -- and even from astronauts on the International Space Station -- to mark the anniversary, which many had feared Mandela would not live to see.

US President Barack Obama -- who was unable to visit Mandela during a trip to South Africa last month -- led tributes to the peace icon, calling on people to honour him through volunteer work.

"Our family was deeply moved by our visit to Madiba's former cell on Robben Island during our recent trip," Obama said in a statement.

"We will forever draw strength and inspiration from his extraordinary example of moral courage, kindness, and humility."

Other well-wishers included the Dalai Lama, former US president Bill Clinton and his wife Hillary, US actor Morgan Freeman and Mandela's former jailer FW de Klerk, who went on to share the Nobel Peace Prize with him.

"Mandela's place in South Africa's history is assured," former president De Klerk said in a statement.

"His legacy of courage, perseverance and magnanimity will continue to inspire us -- and people throughout the world -- for generations to come."

Across the country, biker gangs cleaned streets, volunteers painted schools and politicians spent 67 minutes on worthy projects -- all to mark Mandela's 67 years of public service.

"Let us return Madiba's sacrifices and contributions through our own efforts to build a better society," President Jacob Zuma said.

Near Pretoria, Zuma himself will later try to channel Mandela's cross-community appeal by delivering government housing to poor whites.

After doing their bit, Mandela's family plans to gather at the hospital for lunch, along with Mandela's third wife Graca Machel, who also celebrates 15 years of marriage to her husband today.

"The bulk of the grandchildren will be in Mamelodi at a home for homeless children," Mandela's granddaughter Ndileka said.

"We're doing our 67 minutes and bringing our old clothes that we're not using anymore. Then we'll converge at the hospital to have lunch with granddad."

She said the birthday meal would include Mandela's favourite food, including "oxtail, prawns, dumplings and vegetables".

-- 'Make the world a better place' --

The United Nations declared the Nobel Peace laureate's birthday Mandela Day in 2010, but for many this year it takes on extra poignancy.

In central Lisbon the Don Pedro IV Square was to be renamed Nelson Mandela Square.

On Saturday, the Australian city of Melbourne will hold a concert featuring local and African artists, while a music festival later this year in Norway will promote equality in schools.

Born on July 18, 1918, Mandela fought against white rule in South Africa as a young lawyer and was convicted of treason in 1964.

He spent the next 27 years in jail.

It was in part through his willingness to forgive his white jailers that Mandela made his indelible mark on history.

After negotiating an end to apartheid, he became South Africa's first black president, drawing a line under centuries of colonial and racist suppression.

He then led reconciliation in the deeply divided country.

Mandela's peace-making spirit has won him worldwide respect.

"Never before in history was one human being so universally acknowledged in his lifetime as the embodiment of magnanimity and reconciliation as Nelson Mandela," said retired archbishop Desmond Tutu, himself a Nobel Peace laureate.

But the sunset of Mandela's life has been somewhat eclipsed by bitter infighting among his relatives.

A row over his final resting place has seen three of his children's graves dug up and their remains moved amid public brawling and legal action among his children and grandchildren.