Wife with former leader as he is treated for recurring lung infection, but he is stable, conscious and can breathe unaided

It has become an increasingly frequent occurrence but each time the anxiety deepens rather than diminishes. At 1.30am on Saturday, when the affluent suburbs of Johannesburg were dark and deathly quiet, Nelson Mandela was whisked from his home to hospital with a recurring lung infection.

South Africa's first black president, who turns 95 next month, is said to be in a serious condition – a rare choice of word for government officials who have played down past health scares.

But they also insisted that he is stable, conscious, able to breathe on his own and believed to be communicating with his family. His wife Graça Machel, who on Thursday cancelled a trip to a hunger summit in London, accompanied him to hospital and remains at his bedside.

This is Mandela's third hospitalisation of the year. It has become a solemnly familiar ritual with media gathering outside hospital and home, forensic scrutiny of government communications, a smattering of reckless tweets and a collective shudder among millions of South Africans. Each time old age and mortality are pitched against 21st-century medical science and one man's astounding physical and mental resilience.

Mandela was taken from his home in Houghton, an upmarket suburb of Johannesburg, to a hospital in the administrative capital, Pretoria.

"During the past few days, former president Nelson Mandela has had a recurrence of lung infection," said a statement from the office of president Jacob Zuma on Saturday. "This morning at about 1.30am, his condition deteriorated and he was transferred to a Pretoria hospital. He remains in a serious but stable condition."

The presidency said Mandela was receiving expert medical care and "doctors are doing everything possible to make him better and comfortable".

Zuma wished Mandela a quick recovery on behalf of the government and the nation, and requested that the media and the public respect the privacy of the former leader and his family.

Long retired from politics, Mandela has been vulnerable to respiratory problems since contracting tuberculosis during his 27-year imprisonment under racial apartheid. Last December he was treated for a lung infection and had a procedure to remove gallstones during nearly three weeks in hospital.

He was last discharged from hospital on 6 April after doctors drained fluid from his lung area after diagnosing him with pneumonia.

Speaking last week, before the hospitalisation, his long-time friend and lawyer George Bizos said he had seen Mandela a couple of weeks ago. "He's OK. His memory is not what it ought to be. He's conscious."

Zuma's spokesman Mac Maharaj gave an unusually frank assessment of his condition on Saturday. "I think it's a very balanced picture that he is 95 [in July], it's a recurring illness, he's been treated successfully for it before," he told Sky News. "There are no guarantees for ever. And right now what they [the doctors] are saying is he's stable.

"I've questioned them and I've asked as a layman and they said, 'Look, he's able to breathe on his own.' I think that's important. So I don't want to put a rosy picture and an optimistic picture, nor do I want to paint a bad picture and let's give up all hope."

Maharaj, who was imprisoned with Mandela on Robben Island, said the Nobel peace laureate is conscious and thought to be communicating with his family. "He's a fighter. Let's just face it, he's been through this many times and he's been through worse issues. He's survived. I think at his age his willpower is strong enough.

"I think for all of us who are concerned, I understand that our admiration, our hopes for him, we'd like him to be with us for ever, but we also know that like all human beings, our life is transient and therefore with our prayers, with our thoughts, I know we will help him to be strong."

He described Machel's presence as "a source of strength and comfort" but declined to confirm that other family members had visited.

In April, South Africa's public broadcaster showed footage of a visit by Zuma and other ANC leaders to Mandela at his Johannesburg home. There was an outcry from some viewers who complained that Mandela looked unwell and unsmiling and was being exploited.

Maharaj insisted: "There are moments when he looks terribly frail and there are other moments, within minutes, he changes and you think he looks completely normal and usual."

Mandela's condition triggered expressions of concern in South Africa and around the world. David Cameron tweeted: "My thoughts are with Nelson Mandela, who is in hospital in South Africa."

But Nick Griffin, chairman of the British National Party, caused widespread anger on Twitter by posting: "Saint #nelsonmandela on last legs it seems. Make sure to avoid BBC when the murdering old terrorist croaks. It'll be nauseating."

When challenged, he tweeted in response: "'Statesmen' must be judged on results not rhetoric. Before Mandela, South Africa was a safe economic powerhouse. Now crime-ridden basket case."

A BNP spokesman confirmed that the tweets were genuine.

South Africa's governing African National Congress (ANC) called for mandela to be given privacy and respect.

"As the ANC, we will keep president Mandela and his family in our thoughts and prayers at this time and call upon South Africans and the peoples of the globe to do the same for our beloved statesman and icon, Madiba," spokesperson Jackson Mthembu said.

"We send to him our well wishes for a speedy recovery so that he may soon be discharged to return to the care and comfort of his home."Mandela's last public appearance was at the 2010 football World Cup final in Johannesburg, when Machel supported his arm as he tried to wave to the crowd. Since his withdrawal from public life, the statesman has divided his time between his Johannesburg and Qunu, the village in Eastern Cape province where he spent his early years.