Thousands of people are arriving at a stadium in Johannesburg for a memorial service for Nelson Mandela.
US President Barack Obama and the UN Secretary General Ban Ki-moon will be among those paying tribute.
The service will be held at the 95,000 capacity FNB stadium, where Mr Mandela made his last public appearance. It will also be shown on big screens at three "overflow" stadiums.
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 Joseph Winter reports from the scene.
Many are wearing Nelson Mandela T-shirts or South African football or rugby tops, and are singing songs from the fight against apartheid, our correspondent adds.
People arrive at the FNB stadium ahead of the national memorial service for Nelson Mandela
The crowds will be addressed by South African President Jacob Zuma and there will be tributes from other heads of state, including President Obama, Brazilian President Dilma Rousseff and Cuban President Raul Castro.
Under Mr Castro's brother, Fidel, Cuba was a staunch critic of apartheid, and Mr Mandela had expressed gratitude for his support.
Four of Mr Mandela's grandchildren are also expected to make speeches, along with Andrew Mlangeni, Mr Mandela's friend and fellow former Robben Island prisoner.
Ban Ki-moon, who will also pay tribute as part of the service, visited the Nelson Mandela Centre of Memory in Johannesburg on Monday.
"The world mourns the loss of Nelson Mandela, one of the towering figures of our time and indeed of any time," he said.
"We join together in sorrow for a mighty loss and in celebration of a mighty life."
British Prime Minister David Cameron will be among those attending Tuesday's memorial. Deputy Prime Minister Nick Clegg and Labour leader Ed Miliband will also attend, along with former British Prime Ministers Gordon Brown, Tony Blair and John Major.
Three former US presidents, George W Bush, Bill Clinton and Jimmy Carter, will also be there.
Francois Hollande of France, Palestinian leader Mahmoud Abbas and India's President Pranab Mukherjee are also on the list, as well as Zimbabwe's President Robert Mugabe.
Leading celebrities in the anti-apartheid movement Peter Gabriel and Bono are expected to attend.
They will join a 95,000-strong crowd at FNB stadium, also know as Soccer City, just outside Johannesburg, near Soweto.
Mr Mandela made his final major public appearance there during the 2010 football World Cup.
The BBC's Mike Wooldridge in Johannesburg says the line-up of world leaders is a sign of how South Africa has seen its partners and place in the world since Nelson Mandela became president.
But, he adds, today is as personal as it is political... a recognition that there are lessons for the wider world in everything Nelson Mandela sought to achieve.
'Beautiful send-off'
The memorial service is due to start at 11:00 (09:00 GMT) and is expected to last four hours, according to the official programme.
Because public seating is not ticketed, the South African authorities are certain many people will have to be turned away.
So the service will be shown on big screens at three "overflow" stadiums - Orlando, Dobsonville and Rand - as well as at community venues around the country.
Mr Mandela's body will lie in state in Pretoria on the following three days and a state funeral takes place on Sunday in his home village of Qunu in Eastern Cape province.
Two of Mr Mandela's granddaughters, Tukwini and Nandi, told the BBC the family had been working to give him "the beautiful send-off that he deserves".
More than 100 current or former heads of state or government will attend the funeral or the national memorial, according to the South African government.
Among those not attending the memorial events will be Israel's Prime Minister Benjamin Netanyahu, who cited high travel and security costs.
However there will be suspicion that he wishes avoid the potential for anti-Israeli protests, the BBC's Middle East Correspondent Kevin Connolly reports. Israel had extensive links with the old apartheid regime and many South Africans identify strongly with the Palestinian cause, our correspondent adds.
Israeli President Shimon Peres is not attending due to illness, so Knesset speaker Yuli Edelstein will represent the country.
The Dalai Lama will also be among the notable absentees, after twice failing to get a visa to South Africa.
