Nelson Mandela: Jacob Zuma cancels Mozambique trip
Mr Zuma was due to attend a summit in Maputo of the Southern African Development Community (SADC) to discuss regional infrastructure, but pulled out after visiting the 94-year-old former president in hospital late on Wednesday.
"Over the past 48 hours, the condition of former president Madiba has gone down," presidential spokesman Mac Maharaj told state broadcaster SABC, using his clan name.
Mr Maharaj said Mr Mandela's condition remained critical. He declined to comment on reports that he was on life support in the Pretoria hospital where he is being treated for a lung infection, saying his privacy should be respected.
Mr Mandela has already spent 20 days in the hospital, his fourth hospitalisation in six months.
This has forced a growing realisation among South Africans that the man regarded as the father of their post-apartheid "Rainbow Nation" will not be among them for ever.
"Mandela is very old and at that age, life is not good. I just pray that God takes him this time. He must go. He must rest," said Ida Mashego, a 60-year-old office cleaner in Johannesburg's Sandton financial district.
Mr Mandela, South Africa's first black president, is admired around the world as a symbol of resistance to injustice for the way he opposed his country's apartheid system, spending 27 years in jail, more than half of them on notorious Robben Island.
He is also respected for the way he preached reconciliation after the 1994 transition to multiracial democracy following three centuries of white domination.
Well-wishers' messages, bouquets and stuffed animals have piled up outside Mandela's Johannesburg home and the wall of the hospital compound where he is being treated in the capital.
As they headed to work on Thursday, South Africans seemed resigned to the prospect of losing their hero.
"We are all going to feel bad when he passes, but at the same time we will be celebrating his life. He has done so many great things for this country," said John Ndlovu, a 25-year-old office worker.
Mandela stepped down in 1999 after one five-year term in office.
Since then he has played little role in public life, dividing his time in retirement between his home in the wealthy Johannesburg suburb of Houghton and Qunu, the village in the impoverished Eastern Cape province where he was born.
The public's last glimpse of him was a brief clip aired by state television in April during a visit to his home by Zuma and other senior officials from the ruling African National Congress.
At the time, the 101-year-old liberation movement assured the public Mandela was "in good shape", although the footage showed a thin and frail old man sitting expressionless in an armchair.