Police officers operate like a gang to defend themselves against being investigated, head of South Africa's police watchdog, Robert McBride told MPs on Friday.

"Police protect each other. They function like a gang," McBride said while briefing Parliament's portfolio committee on the annual performance plan of the Independent Police Investigative Directorate (Ipid).

"You will see as more and more of these cases emerge and more is revealed, you will see the extent to which racketeering is the way things are done."

He said corruption within the South African Police Service was the biggest threat to national security as officers "sit with sensitive information", hinting that if offered money, corrupt officers "willing to sacrifice principles" could sell this data to foreign agencies.

McBride said Ipid's senior officers have been threatened "to make us look the other way".

"We have suffered for doing things the right way," he said, adding a warning to those who try to corrupt Ipid officials: "It's not going to work."