Tens of thousands of people queued to file past Mandela's body, lying in state, at the Unions Buildings, where he assumed charge as the country's first black and freely elected president in 1994.
Someone took a picture of Mandela, lying in state, and posted it on Twitter. When the news of the image broke this morning, many believed it was a hoax because of the tight security around Mandela's body, The Star reported.
Mandela died on December 5 at the age of 95 after a protracted illness.
Most people reacted angrily to the picture by asking how it was possible to take a photograph of Madiba (Mandela's clan name) when there was such tight security.
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The government said it is shocked at the blatant breach of security.
"Government is aware of social media activity on the existence or otherwise of a picture of President Mandela lying in state," an official statement said.
Twitter would immediately be asked to withdraw it, she said.
Mandela family spokesman, Lieutenant-General Themba Mathanzima said that the family were very concerned about the picture.
"We do not know who did this, but we are going to be asking the government to investigate," Mathanzima said.
He said the public had been warned not to take any images of Mandela's body.