Thousands of people are gathering at a stadium in Johannesburg for a memorial service for Nelson Mandela.

US President Barack Obama, Cuban President Raul Castro and UN Secretary General Ban Ki-moon will address the service, as will four of Mr Mandela's grandchildren.

The service at the 95,000-capacity FNB stadium is scheduled to start soon but people are still trying to enter.

The former South African president died aged 95 last Thursday.

The country is observing a series of commemorations leading up to the funeral on Sunday.

'A mighty life'

The memorial service is one of the biggest gatherings of international dignitaries in recent years.

Many people stood in the rain waiting for several hours to get into the stadium, the BBC's Pumza Fihlani reports from the scene.

She says the crowds are in high spirits - singing and dancing, stomping their feet - and the stadium is beginning to have the feel of a political rally.

One of those attending, Shahida Rowe from Johannesburg, told the BBC: "The core of Mandela's life was humanity. That is why I am here today and the world is celebrating.

"Thanks to him, I was recognised as a human being."

Mr Mandela's widow, Graca Machel, has now arrived at the stadium - with huge cheers as she was shown on the big screen.

South African President Jacob Zuma will make the keynote address, and other tributes will come from Brazilian President Dilma Rousseff, Chinese Vice-President Li Yuanchao, President Hifikepunye Pohamba of Namibia and Indian President Pranab Mukherjee.

Raul Castro's speech reflects the fact that under his brother, Fidel, Cuba was a staunch critic of apartheid, and Mr Mandela had expressed gratitude for that support.