Mandela's memorial service on Tuesday at FNB Stadium in Johannesburg is expected to be one of the largest such gatherings in generations.
South African President Jacob Zuma announced the mourning period after Mandela died yesterday at the age of 95 following a protracted illness.
Mandela's body will lie in state at the seat of government, the Union Buildings in Pretoria, for three days before the state funeral in his childhood home of Qunu on December 15.
Sunday, December 8, has been declared as a national day of prayer and reflection.
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Memorial events for the statesman who dismantled the apartheid regime and served as South Africa's first black President will be held across the country during December 11-13.
"We should all work together to organise the most befitting funeral for this outstanding son of our country and the father of our young nation," said a statement from the presidency.
Leaders from across the world are expected to pay their respect to Mandela and at least three American presidents, including Barack Obama, will travel to South Africa to attend memorial events.
White House spokesman Jay Carney said Obama and the First Lady would "participate in memorial events", without giving details. The Obamas will be joined on Air Force One by former President George W Bush and wife Laura Bush.
Preparations to bid adieu to Mandela were underway in his childhood village of Qunu, where residents were awaiting the final return of their beloved son.