Williams, 51, was killed by lethal injection in a Californian prison on Tuesday for four murders during robberies in 1979.
His death sentence sparked outrage after he picked up a Nobel Peace Prize nomination for his death row writings, which urged young people not to form gangs.
Williams' celebrity supporters campaigned relentlessly for clemency ahead of the execution and many, including Snoop Dogg and human rights activists Jesse Jackson, are tipped to speak at his funeral.
The ceremony will be held in Los Angeles, before his ashes are scattered in South Africa.
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