Nelson Mandela on life support

"Yes, he is using machines to breathe," Napilisi Mandela told AFP late on Wednesday after he saw the 94-year-old in the Pretoria hospital where he has been treated for a recurrent lung infection for nearly three weeks.

"It is bad, but what can we do," added the elder who usually presides over family rituals and meetings.

President Jacob Zuma late Wednesday abruptly cancelled a trip to neighbouring Mozambique after he visited Mandela, who has been in critical condition for several days.

It is the first time Zuma has scrapped a public engagement since Mandela was hospitalised on June 8.

"President Zuma was briefed by the doctors who are still doing everything they can to ensure his well-being," a statement from the presidency said.

Twenty hours earlier Cape Town Archbishop Thabo Makgoba had visited the Mediclinic Heart Hospital to pray with wife Graca Machel "at this hard time of watching and waiting".

"Grant Madiba eternal healing and relief from pain and suffering," the prayer said, using Mandela's clan name by which he is fondly known. "Grant him, we pray...a peaceful, perfect, end."

Outside the hospital emotional crowds have been holding vigils, offering their own prayers and remembering the life of one of the greatest figures of the 20th century.

Supporters sang hymns for the father of South African democracy and the architect of remarkable transition from almost half a century of white minority rule to landmark multiracial elections.

"We have been so united -- blacks and whites together. That's the thought of Mandela in us," said Lerato Boulares, 35.

With his life seemingly slipping away, messages of support for the former president blanketed a wall outside the hospital, including a poster bearing one of his most memorable quotes: "It only seems impossible until it's done".

Mandela's lung troubles date from his 27 years locked up on the notorious Robben Island and in other apartheid prisons.

Elders from Mandela's Thembu clan visited the country's first black president as his "Rainbow Nation" comes to terms with the increasing frailty of the man who defeated decades of racist white minority rule to become the country's first black president.

Zuma led delegates at a union conference in a rousing song Wednesday evoking Mandela's role as a moral compass and leader of the struggle for freedom.

The South African president said that Mandela had spent his life "in dedication to humanity".

Meanwhile messages of goodwill flooded in from overseas.

UN leader Ban Ki-moon said the whole world was praying for "one of the giants of the 20th century".

"I know our thoughts and prayers are with Nelson Mandela, his family and loved ones, all South Africans and people across the world who have been inspired by his remarkable life and example," Ban said.

Hillary Clinton offered "love and prayers to our great friend, Madiba, his family and his nation during this difficult time."

The White House has also sent its wishes but could not yet say whether his ill health would affect a planned visit by US President Barack Obama to South Africa from Friday as part of a tour of Africa.

Mandela's fragile state of health has sparked speculation that the tour could be halted, or radically changed, if the anti-apartheid icon passes away while Obama is on the continent.

South Africa's foreign minister Maite Nkoana Mashabane has said that while Obama would have loved to see Mandela, a meeting with the former South African leader would be impossible.

Mandela -- whose 95th birthday is on July 18 -- has been hospitalised four times since December, mostly for a recurring lung infection dating back to his time in jail for sabotage against the apartheid government.

The world watched as the man once branded a terrorist by the United States and Britain raised his fist as he walked free from prison near Cape Town in 1990.

Mandela went on to negotiate an end to white minority rule and won South Africa's first fully democratic elections in 1994.

He forged a path of racial reconciliation during his single term as president, before taking up a new role as a roving elder statesman and leading AIDS campaigner.

He stepped back from public life in 2004 and has not been seen in public since the football World Cup finals in South Africa in 2010.

With his health now fading, Mandela's supporters are starting to show signs of resignation -- while preparing to celebrate his legacy.