The key address in the memorial service for Nelson Mandela was given by Barack Obama, whose words were brought to life for deaf spectators and TV viewers by a “sign language interpreter”, who could be seen gesturing energetically behind the sombre US President.

Yet the man, not only seen by the tens of thousands in Johannesburg’s FNB stadium where the memorial took place on Tuesday, but also by millions across the world on television, was a “fake”, according to Bruno Druchen, the national director of the Deaf Federation of South Africa.

Mr Duchen told the Associated Press “there was no meaning in what he used his hands for”. He and other language experts pointed out that the man was not signing in South African or American sign languages and could not have been signing in any other known sign language because there was no structure to his arm and hand movements. South African sign language covers all of the country’s 11 official languages.

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Amjad Shaikh, a supporter of the ruling ANC party who studied sign language as a boy, said he tried to contact the organisers of the memorial after seeing the man on television. “What this man was doing was making no sense,” he said.

Mr Sheikh said at first he thought the man may have been using “a foreign system” of sign language that he did not understand, but having asked around, he discovered “no one knew anything about it”.

The Action on Hearing Loss group complained that as a result of what happened, “deaf or hard-of-hearing people across the world were completely excluded from one of the biggest events in recent history”.

Ingrid Parkin, principal of the St Vincent School for the Deaf in Johannesburg, received complaints from deaf people across the world, from Canada to China. “This man himself knows he cannot sign and he had the guts to stand on an international stage and do that,” she said.

She added that bogus sign-language interpreters were a problem in South Africa, “because people who know a few signs try to pass themselves off as interpreters. And those hiring them usually don’t sign, so they have no idea that the people they are hiring cannot do the job”.

The man has so far not been identified. Collins Chabane, one of South Africa’s two presidency ministers, admitted that the government had not finished investigating the matter because it has been overwhelmed by the task of organising the public viewing of Mr Mandela’s body in the South African capital of Pretoria, and his funeral in his home town of Qunu on Sunday. Jackson Mthembu, spokesman for the governing African National Congress party, declined to comment. This was the latest criticism levelled at the South African government for its organisation of 10 days of tributes to the former President and anti-apartheid icon.

Security was also reported to be lax at the memorial, in which few bags of spectators were searched and faulty transport was supplied to carry members of the public trying to attend the event.

Meanwhile, the Cape Town home of retired Archbishop Desmond Tutu was robbed while he was attending the service for Mr Mandela, with whom he worked closely during and after the struggle against apartheid. No arrests have been made.