The national memorial service for former South African President Nelson Mandela has begun about an hour late in Johannesburg.

US president Barack Obama was late for the ceremony due to bad weather conditions in the South African city.

The ceremony started with a choir performance of South African national anthem.

The crowds twice booed South African President Jacob Zuma, who was to give the keynote address at the service. Many South Africans are unhappy with Zuma because of state corruption scandals, though his ruling African National Congress, once led by Mandela, is the front-runner ahead of elections next year.

The memorial is held at the soccer stadium that hosted the opening and closing ceremonies for the 2010 World Cup. There, the former president made his last public appearance.

The FNB stadium in Johannesburg has a capacity of 95 000. The service is also shown on big screens at three "overflow" stadiums.

Despite the heavy rain, thousands of people are expected to attend the memorial for former South African president Nelson Mandela. One-third of the seats at FNB stadium are still empty, and the other three stadiums are yet to be crowded, however mourners keep arriving at the ceremony.

The crowds are in high spirits - singing and dancing, stomping their feet - and the stadium has the feel of a political rally, the BBC reports.

Mandela's widow, Graca Machel, and ex-wife Winnie Madikizela-Mandela arrived at the stadium to huge cheers; the two of them hugged and kissed.

British Prime Minister David Cameron is attending Tuesday's memorial, along with Deputy Prime Minister Nick Clegg, Labor leader Ed Miliband and former British Prime Ministers Gordon Brown, Tony Blair and John Major.

Cameron told the BBC how he had been struck by Mandela's "incredible grace and forgiveness - how he reconciled people after all those years in jail".

Three former US presidents, George W Bush, Bill Clinton and Jimmy Carter, are also attending, as are Francois Hollande of France, Palestinian leader Mahmoud Abbas and Zimbabwe's President Robert Mugabe.

Celebrity mourners on the list include Oprah Winfrey, Bono, Sir Richard Branson and Peter Gabriel.

Dalai Lama will be among the notable absentees, after twice failing to get a visa to South Africa.

In addition to Zuma, US President Barack Obama is also scheduled to speak, along with UN Secretary-General Ban Ki-Moon, Brazilian President Dilma Rousseff and Cuban President Raul Castro.

The memorial service is one of the biggest gatherings of international dignitaries in recent years as it will be attended by almost 100 world leaders

Crowds stood in the rain waiting for several hours to get into the stadium. Many are wearing Nelson Mandela T-shirts or South African football or rugby tops, and are singing songs from the fight against apartheid, the BBC correspondent reports.

The ceremony is aired live by the Bulgarian public TV, BNT, starting at 11 am local time.