Although it had been uncertain whether Madiba would be up to it, Obama was given the go-ahead to visit him right after his wife, Graca Machel, had given her a tour of the Nelson Mandela Foundation's centre of memory - where they keep Mandela's memorabilia.

The foundation CEO Achmat Dangor told journalists: "She [Obama] went to pay a courtesy call to Mandela. We don't comment on his health publicly, but it's clear he's well enough to see her."

The foundation released a picture later in the afternoon of Obama sitting alongside Mandela as they read a book containing some of his most memorable quotes.

Earlier in the day, Obama had both local and international media - even her daughters Malia Ann, 13, and Sasha, 10, (who came in a separate car) - waiting for her arrival to meet Machel.

She had been to the US embassy in Pretoria for a meet-and-greet with staff there before heading to the presidential guest house (also in Pretoria) where she was welcomed by one of President Jacob Zuma's wives, Nompumelelo Ntuli-Zuma.

Back at the foundation, Obama's head of staff, Tina Tchen, took time to look around and read some history on the Rivonia treason trial.

In the foyer some US journalists asked their local counterparts about Mandela's recent public activities and general health.

More than an hour after her daughters, niece Leslie and nephew Avery Robinson (children of Craig Robinson, Obama's brother) had arrived, the US first lady arrived and headed for a private meeting with Machel.

Mandela's daughters, Makaziwe Mandela and Zenani Dlamini, his granddaughters, Ndileka and Zoleka Mandela, Zinhle Nkosi, Swati Dlamini and Zaziwe Mandela, were also present to meet Obama.

They eventually came out to be greeted by media.

Machel first introduced Obama to Thoko Mavuso, the liaison officer in Dangor's office, saying Mavuso had worked under Mandela for 21 years.

As Machel spoke, all eyes were on Obama -who looked stunning in a burnt orange silk two-piece outfit, rounded off with blue pumps and a floral print camisole with her hair impeccably combed back.

Obama then met the foundation's head of the centre of memory, Verne Harris, who subsequently showed the family some artefacts and rare documents of Madiba's prison collection going back into the 1970s.

As they perused the documents, Obama held Sasha on both shoulders and said: "Listen, because you're going to be tested on this."

Obama then asked how Mandela had kept all the documents and journal entries for all these years.

Harris told her Mandela had always had a notepad with him.

Late yesterday, Obama visited a daycare centre in Zandspruit, an informal settlement north of Johannesburg, before visiting the Apartheid Museum in Ormonde, southern Johannesburg, where she met young women from all over the continent who were hosted by the Young African Women Leaders Forum in Pretoria.

Obama told the women that she wanted her daughters to be like them.

With her back to the cameras and journalists, she told them: "This is just the beginning. This is the start of a conversation with women from all over the globe.

"I'm so proud of you all. This is the culmination of everything that I could imagine. I'm humbled. I just wanna hug you all."

Dianah Mukundwa from Rwanda said of Obama: "She was so inspiring. I expected her to be like a first lady and be about protocol, but she wasn't.

"We asked her how [Barack] Obama felt about her being away, and she told us that he knew she was a strong woman."

Obama also briefly spoke to the group about fitness and the importance of getting enough sleep.

Tomorrow she will visit the Regina Mundi Church in Soweto - a sanctuary for students during the 1976 uprisings.

She is expected to address the youth there, where media will finally have access to her interactions, although no official press conference or interviews will be staged.

She will also visit the Hector Pieterson memorial square in Orlando West, Soweto.