In response to the historic handshake between Barack Obama and Cuban president Raul Castro, the US National Security Council has said: "This wasn't a pre-planned encounter. Above all else, today is about honouring Nelson Mandela and that was the President's singular focus at the memorial service.
"We appreciate that people from all over the world are participating in this ceremony."
President Raul Castro says Nelson Mandela set an "insurmountable example" to Latin America and the Caribbean which he says are "currently moving towards unity and integration for the benefits of their peoples on the basis of respect for diversity".
US President Barack Obama will lead tributes from world leaders to Nelson Mandela tomorrow at the memorial service in Johannesburg.
UN Secretary General Ban Ki-moon will lead the international tributes, followed by Dr Nkosazana Dlamini Zuma from the African Union.
Obama will be followed by Brazil's President Dilma Rousseff, China's Vice-President Li Yuanchao, Namibia's President Hifikepunye Pohamba, India's President Pranab Mukherjee and the last international tribute will come from President Raul Castro.