South African officials are investigating an incident during the fourth day of the first Test against Pakistan in which an unnamed South African allegedly made an insulting remark which was heard by television viewers around the world.

"They're like animals," was the comment picked up by a stump microphone which apparently had not been turned off as normally happens soon after a ball has been bowled.

Mickey Arthur, South Africa's coach, said the comment had been the subject of complaining e-mails, some "from as far afield as Pakistan." He added: "The comment was not aimed at any Pakistan player."

Arthur said the incident was the subject of an internal investigation and that a statement will be issued on Monday. As far as he was aware, he said, it had not become a matter for match referee Chris Broad.

The comment was made shortly before lunch, soon after a complaint from the South African team about three spectators near the boundary who allegedly swore at bowler Paul Harris.

The head of security at the ground, Terence Jacobs, said the three spectators, who were supporters of Pakistan, were questioned and two were asked to leave the ground after admitting that they had sworn at the players.

Arthur added he was not happy about the stump microphones. "They are a bit intrusive," he said. "What is said on the field should stay on the field."