Former South African President Nelson Mandela was admitted to hospital after a recurrence of a lung infection, the South African president's office said Thursday.
The Nobel laureate was admitted to hospital on Wednesday night.
"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," President Jacob Zuma's office said in a statement.
"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 statement said.
Madiba refers to his affectionate clan name, which is widely used in the nation.
Earlier this month, the anti-apartheid icon was hospitalized again, but officials said it was only for a routine checkup.
Mandela, 94, has undergone treatment for the ailment before, including a Christmas hospital stay last year for gallstones surgery and a lung infection. He was also treated for an acute respiratory infection in 2011.
The former president suffered from tuberculosis while imprisoned, and has battled respiratory infections over the years.
Considered the founding father of South Africa's democracy, Mandela became an international figure when he endured 27 years in prison for fighting racial segregation.
He was freed in 1990. Three years later, he and then-South African President F.W. de Klerk won the Nobel Peace Prize.
In 1994, Mandela was elected president, serving only one term as he had promised.
Despite his rare public appearances in recent years, he retains his popularity and is considered a hero of democracy in the nation. Last year, South Africa launched a new batch of banknotes with a picture of a smiling Mandela on the front, a testament to his iconic status in the nation.
Mandela's impact extends far beyond South African borders. After he left office in 1999, he was involved in mediating conflicts from Africa to the Mideast.