Nelson Mandela, the anti-apartheid icon who became South Africa's first black president, is "responding positively" to treatment for a recurring lung infection, a presidential office statement has said, as his supporters held prayers for his recovery.
President Jacob Zuma has sought to reassure South Africans that Mandela was in good hands as his doctors reported some progress in his treatment.
Mandela, 94, was admitted to hospital overnight. Authorities declined to name the hospital where he is receiving treatment but he is widely believed to be either at Mediclinic Heart Hospital or 1 Military Hospital - both in Pretoria.
"He remains under treatment and observation in hospital," the statement said on Thursday, without giving further details.
The Nobel peace laureate was conscious when he was admitted, Zuma's spokesman Mac Maharaj told AFP news agency.
Mandela has become increasingly frail in recent years and has been admitted to hospital several times since last year.
He was hospitalised earlier this month, receiving what a presidential spokesman described as a "successful" medical test.
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"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," the statement from Zuma's office said.
It appealed "for understanding and privacy in order to allow space to the doctors to do their work".
Zuma wished Mandela a speedy recovery, referring to him affectionately by his clan name, "Madiba".
"We appeal to the people of South Africa and the world to pray for our beloved Madiba and his family and to keep them in their thoughts," the presidential statement quoted Zuma as saying.
"We have full confidence in the medical team and know that they will do everything possible to ensure recovery."
The series of hospitalisations has seen an outpouring of prayers, but has also seen South Africans come to terms with the mortality of their national hero.
Al Jazeera's Haru Mutasa, reporting from Johannesburg, said many South Africans were concerned about Mandela's health.
"There is a lot activity on social media - Facebook and Twitter. Many people wishing him well, hoping that he will soon be discharged," Mutasa said.
"Most South Africans understand the fact that he is an old man, and that he will go in and out of hospital more often than not. There isn't a sense of panic right now."
Barack Obama, the US president, sent his best wishes to the democracy icon.
"He is as strong physically as he's been in character and in leadership over so many decades, and hopefully he will ... come out of this latest challenge," Obama told reporters at the White House.
"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," Obama said.
Mandela spent a night in a hospital and was released on March 10 following a medical test. At that time, presidential spokesman Mac Maharaj said the former president was "well".
In December, Mandela spent three weeks in a hospital, where he was treated for a lung infection and had a procedure to remove gallstones.
A year ago, Mandela was admitted to a Johannesburg hospital for what officials initially described as tests but what turned out to be an acute respiratory infection.
He was discharged days later. He also had surgery for an enlarged prostate gland in 1985.
Under South Africa's white-minority apartheid regime, Mandela spent 27 years in prison, where he contracted tuberculosis, before being released in 1990.
Mandela is idolised in his home nation, where he is seen as the architect of the country's peaceful transition from apartheid state to democracy despite fears of much greater bloodshed.
He served one five-year term as president before retiring.