A SOUTH African sign language interpreter accused of miming nonsense as world leaders paid tribute to Nelson Mandela has defended himself as a "champion" signer, but said he suffered a schizophrenic episode at the time.

The interpreter, Thamsanqa Jantjie, 34, told Johannesburg's Star newspaper he started hearing voices and hallucinating while on stage, resulting in gestures that made no sense to outraged deaf people around the world.

"There was nothing I could do. I was alone in a very dangerous situation. I tried to control myself and not show the world what was going on. I am very sorry. It's the situation I found myself in," he told the paper.

The government admitted Mr Jantjie was not a professional interpreter but played down security concerns at his sharing the podium with world leaders including US President Barack Obama at the memorial on Tuesday.

"He was procured. He did not just rock up," Deputy Disabilities Minister Hendrietta Bogopane-Zulu told a news conference. "Did a mistake happen? Yes. He became overwhelmed. He did not use the normal signs. We accept all that."

South Africa's leading deaf association denounced him as a fake, making up gestures for Mr Obama and South African president Jacob Zuma's speeches.

Mr Jantjie said he did not know what triggered the attack and said he took medication for his schizophrenia.