Local interpreter Francois Deysel said the man was "moving his arms (to) try look busy" in an "embarrassing" spectacle.
Nicole Du Toit, another official sign language interpreter who also watched the broadcast, said in a telephone interview that the man on stage purporting to sign was an embarrassment.
"It was horrible, an absolute circus, really really bad," she said. "Only he can understand those gestures."
South African parliamentarian Wilma Newhoudt-Druchen, a deaf member of the ruling party, also said the man communicated nothing with his hand and arm movements.
South African sign language covers all of the country's 11 official languages, according to the federation.
It wasn't immediately clear if the unidentified man was using a different method to communicate.
Loening said her organisation was getting mail from around the world "wondering what on earth this man was doing there".
"It's a real embarrassment. It's complete disrespect for the deaf community and for what Nelson Mandela stood for and the support which he gave toward the deaf community."
Jackson Mthembu, spokesman for the governing African National Congress party, declined to comment. "Government will be able to assist you," Mthembu said.
The South African government is preparing a statement.
A bold performance... particularly from the interpreter, it seems. Picture: APSource: AP
-- with AAP and AFP