JOHANNESBURG, South Africa – World leaders and joyous, singing South Africans gathered Tuesday to honour Nelson Mandela at a Soweto soccer stadium that was half full, amid cold, driving rain.

The crowds twice booed South African President Jacob Zuma, who was to give the keynote address at the service, which was an hour late. It was supposed to start at 11 a.m. local time (4 a.m. ET), but started just after noon (5 a.m. ET).

Crowds converged on FNB Stadium in Soweto, the Johannesburg township that was a stronghold of support for the anti-apartheid struggle that Mandela embodied as a prisoner of white rule for 27 years and then during a peril-fraught transition to the all-race elections that made him president.

Steady rain kept many people away. Shortly before the start of the ceremony, the 95,000-capacity stadium was about 50 per cent full. The ceremony began with the singing of the national anthem.

The mood, though, was celebratory. A dazzling mix of royalty, statesmen and celebrities was in attendance.

Thabo Mbeki, the former South African president who succeeded Mandela, got a rousing cheer as he entered the stands. French President Francois Hollande and his predecessor and political rival, Nicolas Sarkozy, arrived together. United Nations Secretary-General Ban Ki-moon waved and bowed to spectators who sang praise for Mandela, seen by many South Africans as the father of the nation.

Mandela died Dec. 5 in his Johannesburg home at the age of 95.

Mandela’s widow, Graca Machel, was at the stadium. So were actress Charlize Theron and singer Bono.

Tuesday was the 20th anniversary of the day when Mandela and South Africa’s last apartheid-era president, F.W. de Klerk, received the Nobel Peace Prize for their efforts to bring peace to their country.

Mandela said in his acceptance speech at the time: “We live with the hope that as she battles to remake herself, South Africa will be like a microcosm of the new world that is striving to be born.”

People blew on vuvuzelas, the plastic horn that was widely used during the World Cup soccer tournament in 2010, and sang songs from the era of the anti-apartheid struggle decades ago.

The 95,000-capacity soccer venue was also the spot where Mandela made his last public appearance at the closing ceremony of the World Cup. After the memorial, his body will lie in state for three days at the Union Buildings in Pretoria, once the seat of white power, before burial Sunday in his rural childhood village of Qunu in Eastern Cape Province.

Obama urges world to act on Mandela’s legacy

At the memorial, President Barack Obama — who landed in South Africa early Tuesday — said Mandela earned his place in history through struggle, shrewdness, persistence and faith. He compared the former South African president to Mahatma Gandhi, Martin Luther King Jr. and Abraham Lincoln.

He praised Mandela as the last great liberator of the 20th century, urging the world to carry on his legacy by fighting inequality, poverty and discrimination.

“For nothing he achieved was inevitable,” Obama said. “In the arc of his life, we see a man who earned his place in history through struggle and shrewdness, persistence and faith. He tells us what’s possible not just in the pages of dusty history books, but in our own lives as well.”

At the memorial service, Obama shook hands with Cuban President Raul Castro, which came during a ceremony that’s focused on Mandela’s legacy of reconciliation.

Eulogies were also to be delivered by U.N. Secretary-General Ban Ki-moon, Chinese Vice-President Li Yuanchao and Castro, the leaders of Brazil, Namibia and India, as well as tributes from Mandela’s grandchildren. Zuma was to give the keynote address.

-Associated Press writers Jon Gambrell, Alan Clendenning and Julie Pace, and Toronto staff