South African police have threatened a Johannesburg Twitter user with arrest for using the social media site to tweet regular updates on police road blocks and speed cameras.

To his fans, the anonymous PigSpotter is a Robin Hood figure fighting police corruption by warning citizens of the locations where police are stopping car drivers around the city, allegedly in some cases, extracting bribes.

To the police, however, his warnings make their job harder they say, altering drunk and speeding drivers to legitimate police checks.

PigSpotter’s tweets document a near-realtime list of claimed police actions, much of which resembles policing the world over.

“Speed traps both directions this morning on Hans Schoeman cnr Silverpine,” read one of PigSpotter’s tweets from this weekend. “Metro pulling off for seatbelts and cellphones at Woodmead onramp going north. Phones down, seatbelts on, Lights on and reduce speed,” read another.

What appears unusual, however, is the volume of police traps, which fuels the suspicion among PigSpotter’s supporters that officers have ulterior motives in stopping people.

South African police say they have now opened a case against the unnamed individual behind the persona on the grounds of ‘defeating the ends of justice’.

"We have information we are following up. He will face the consequences of his actions," police spokesman Wayne Minaar was reported to have told journalists. "The criminal case will now run its course and there will be no further comment," he said.

What has baffled some is the high degree of accuracy of his reports and the speed with which he posts them, creating the impression that he might be being aided by police informants.

The police case is likely to draw more attention to the cause PigSpotter claims to be fighting for, so he might yet achieve his aims even if prosecuted.  In a matter of days his follower base on Twitter has risen from 14,000 to nearly 17,000 and rising.