Deaf actress Marlee Matlin has blasted the organisers of Nelson Mandela's memorial service in South Africa for allowing a mystery man to take to the stage and perform fake sign language during the speeches.

Officials are still trying to figure out how the bogus interpreter was allowed to get so close to world leaders at Tuesday's (10Dec13) send-off, and the Oscar winner insists that whoever is responsible for the gaffe should be reprimanded.

Matlin, who was watching the memorial service in Los Angeles early on Tuesday and trying to make sense of what sign language she saw, tweeted, "Fake 'interpreter' makes up sign language during #Mandela service. Unbelievable!"

And on Wednesday (11Dec13), the Children of a Lesser God star told Entertainment Tonight, "He wasn't doing anything that was close to sign language. It's so degrading... I couldn't even start to imitate it."