* Obama wishes Mandela well
* President Zuma says country "must not panic"
* Mandela spent nearly three weeks in hospital in December
Nelson Mandela spent a second night in hospital being treated for a lung infection while the South African government sought to reassure the nation about the health of its first black president and hero of the anti-apartheid struggle.
The 94-year-old Nobel Peace Prize laureate received well wishes from global figures including U.S. President Barak Obama after he was admitted to hospital before midnight on Wednesday, his third stint in hospital in four months.
The government said on Thursday he was responding well to treatment but had no new statement on his condition as of Friday morning.
Current President Jacob Zuma urged the nation to remain calm and has asked people across South Africa and the world to pray for him.
"Of course I have been saying to people, you should bear in mind Madiba is no longer that young and if he goes for check-ups every now and again, I don't think people must be alarmed about it," Zuma told the BBC on Thursday.
"I would like to really say the country must not panic."
Madiba is the clan name by which many South Africans refer to Mandela.
Mandela has been mostly absent from the political scene for the past decade but remains an enduring and beloved symbol of the struggle against racism.
He is revered at home and abroad leading the struggle against white minority rule - including spending 27 years in prison on Robben Island - and then promoting the cause of racial reconciliation.
He became South Africa's first black president after winning the country's first all-race election in 1994.
U.S. President Obama sent Mandela his best wishes.
"When you think of a single individual that embodies the kind of leadership qualities that I think we all aspire to, the first name that comes up is Nelson Mandela. And so we wish him all the very best," he said.
Mandela was in hospital briefly earlier this month for a check-up and spent nearly three weeks in hospital in December with a lung infection and after surgery to remove gallstones.
That was his longest stay in hospital since his release from prison in 1990 after serving almost three decades for conspiring to overthrow the apartheid government.
Mandela has a history of lung problems dating back to when he contracted tuberculosis as a political prisoner.
As he has receded from public life, critics say his ruling African National Congress (ANC) has lost the moral compass he bequeathed it when he stepped down as president in 1999.
Under such leaders as Mandela, Walter Sisulu and Oliver Tambo, the ANC gained wide international respect as it battled white rule.
Once the yoke of apartheid was thrown off, it began governing South Africa in a blaze of goodwill from world leaders who viewed it as a beacon for a troubled continent and world.
Almost two decades later however, this image has dimmed as ANC leaders have been accused of indulging in the spoils of office, squandering mineral resources and engaging in power struggles.
Mandela spent much of last year in Qunu, his ancestral village in the poor Eastern Cape province. But since his release from hospital in December he has been at his home in an affluent Johannesburg suburb, closer to sophisticated medical facilities.