LIVE COVERAGE; World Unites For Nelson Mandela Memorial.
Obama and his wife Michelle stepped off Air Force One after a 16-hour flight at Waterkloof air force base near Pretoria.
Obama, holding a black umbrella over the first Lady, was followed down the steps by former president George W. Bush and his wife Laura.
Two other former presidents, Bill Clinton and Jimmy Carter, were arriving separately.
Tens of thousands of South Africans converged in sorrow, celebration and pride Tuesday on Soweto's World Cup stadium, to sit side-by-side with presidents, priests and queens at a memorial service for unifying global icon Nelson Mandela.
Close to 100 world leaders were among 80,000 people expected to cram into the venue in Soweto - the crucible of Mandela's anti-apartheid struggle - to bid farewell to a man whose life story earned uncommon universal respect.
Africa was represented by Nigeria's Goodluck Jonathan and more than a dozen other heads of state and government.
Talk show queen Oprah Winfrey and singer-activist Bono, as well as British billionaire Richard Branson and musician Peter Gabriel were expected to be among the celebrity mourners.
In the Soweto stadium, a radiant Musa Mbele, his clothes drenched by the rain declined the offer of an umbrella.
"If he was able to stay behind bars for 27 years for us, what is one day, just one rain-drenched day?" he said.
