A mystery South African man who acted as a sign-language interpreter at Nelson Mandela's memorial service was a "fraud" who simply made "childish hand gestures" for hours as he stood on stage.

Deaf groups say the man, who has not yet been identified, made no sense in any language, and did not seem to know the recognised signs for South Africa, Mr Mandela's clan name Madiba, President Jacob Zuma or Thabo Mbeki, the former president. As a result, they say, he had the effect of marginalising the deaf community, which was "contrary to everything Mandela fought for".

The revelation also raises questions about the security at the event, which was attended by 91 heads of state and government members, including Barack Obama and David Cameron. The interpreter, who wore the clearance pass of a government official, stood feet from Mr Obama as the US president made his speech, and also interpreted for Mr Zuma, who apparently faces such a high threat level that he recently spent pounds 12.4?million on security at his home.

David Buxton, the CEO of the British Deaf Association, called on the South African authorities to "name and shame" the man who, he said, had acted in a way that was "disrespectful and hurtful" to deaf people around the world. While some South Africans took to Twitter to claim that the man had been signing in a native language such as Xhosa or Zulu, Mr Buxton said he was making "childish hand gestures and clapping - it was as if he had never learnt a word of sign language in his life". "It was hours of complete nonsense," he said.

"He is clearly a fraud who wanted to stand on stage with big and important people. It is incredibly disrespectful and hurtful to the deaf community." Mr Buxton said the man had provided sign language for a speech for Mr Zuma at a military event last year. At that appearance, a deaf person in the audience videotaped the event and gave it to the Deaf Federation of South Africa (DeafSA), which submitted a formal complaint to the African National Congress. Sign language experts in South Africa said that all of the country's 11 official languages were covered by the same signs, and they saw none of them used, nor any of the facial gestures that usually feature. Ingrid Parkin, the principal of the St Vincent School for the Deaf in Johannesburg, said she had received complaints about the "gibberish" interpretation.

"This man himself knows he cannot sign and he had the guts to stand on an international stage and do that," she said. Martie Miranda, a sign language instructor at the University of the Free State, said the interpreter marginalised deaf viewers and was "contrary to everything Mandela fought for". It remains unclear whether the man was called upon by the ANC government, which organised the event in Johannesburg, or the SABC, the national broadcaster. The government said it was looking into how the man was recruited.

The SABC could not be reached for comment. The ANC confirmed that it had used him "as a volunteer" at several events previously, including its centenary celebrations in Bloemfontein last year. "We've never had any complaints before," said Keith Khoza, a spokesman. But Wilma Newhoudt-Druchen, an ANC MP and the vice-chairman of DeafSA, told the City Press newspaper that the federation had submitted a report about the man to the party in 2012 but received no response. "When a deaf person complains, nobody listens," she said.

The embarrassment of the interpreter was compounded by the news that Archbishop Emeritus Desmond Tutu's house in Cape Town was burgled as he spoke at Mr Mandela's memorial service. The Nobel Peace laureate, the retired Anglican archbishop of Cape Town, used the pulpit to preach against apartheid and was a contemporary of Mr Mandela.

"I can confirm the house was broken into while he was in Gauteng with his family. No one was at home," said Roger Friedman, a Tutu family spokesman, referring to the province that includes Johannesburg. Police have yet to determine what, if anything, was stolen.