A throwaway remark, a four-word statement and 40 T-shirts. That was all it took to trigger a vicious debate about guilt, responsibility and race that has revealed South Africans at their best – and their worst.

It came about when Roger Young, a film-maker and writer, was discussing a recent controversy about a supermarket with job vacancies that targeted black recruits, prompting a white outcry. "I said to a friend: "I'm going to get all these people a T-shirt saying, 'I benefited from apartheid', because they simply don't understand," he recalled.

The friend took him at his word and together they printed 10 such T-shirts and displayed them in an art exhibition under a sign which said: "Free T-shirts, whites only." They were gone in five minutes, so 30 more were produced and sold not for profit.

Some praised the gesture as honest and courageous in a country that, despite its official aspiration to non-racialism, is still steeped in the legacy of apartheid 18 years on – recent census figures showed that black workers earn six times less than their white counterparts. Others, however, unleashed a tirade of inflammatory criticism and personal abuse that suggested attitudes no less stagnant.

"I tell you who benefited from apartheid, it was blacks," wrote Facebook user Margarita Barnard. "I wish blacks would give whites apartheid. And I will tell you why I say this. Whites came to a country where there was nothing, just some black tribes living in mud huts killing each other. No roads no infrastructure no South Africa even. Blacks were always dying from famines when there were droughts, from tsetse fly [sleeping sickness], from yellow fever, malaria, name it they died in droves."

She continued: "They had no doctors, no writing, no schools no hospitals no roads, and worst of all and something which probably cause more deaths than the rest, no sewage system. Whites came and provided all those at the expense of whites, white know how gave blacks everything they take for granted today. Like clothes, pens, computers, everything of a billion things it needs to create a civilization. BUT whites couldn't civilise them, so apartheid was necessary to keep whites alive."

Another objector, Francois DeWet, posted: "Get me a dictionary or something that shows me how black Africans could be taught in their own languages subjects like maths, science and biology. Simple, you cannot teach in a language that does not have the terminology to do so. We didn't place restrictions on the development of their languages and simply had to find another way to give them a start in live. So, alternative mediums were introduced to accommodate the lack of terminology, and they went apeshit!!"

Alan Marsden wrote: "Is there a punchline to this joke? The fact that like all colonial powers we found a race entrenched in the iron age and lifted them out of it with technology, medicine and education does not count? ... No, I don't feel guilty. In fact I am well annoyed that what we built has gone to wrack and ruin in incapable hands (allegedly the fault of apartheid, even though most of Africa STILL live in the iron age, and apparently like it)."

Such was the hostility on one website that Young's collaborator, Leonard Shapiro, was moved to comment: "It is very seldom that I come across a chat room with so many people full of bitterness and fear."

Young, 40, has also been dismayed by the backlash that has included hate mail, mostly from South African migrants in Britain or Australia. "It's been quite rough," he said. "There was a guy in the UK saying 'You don't know what you're talking about and all the people are going to die.' I got conspiracies on Facebook saying the government is collaborating with China to carry out a white genocide.

"The comments have said the time for white guilt is over. I don't think guilt is helpful but it's really not what this is about. For me it's an economic issue."

He refuses to believe, however, that his spontaneous campaign has exposed something ugly in a generic white psyche. "I think it's a small but vocal minority. You've got people up against the wall who don't understand what's happening."

But he warned: "There are people who say, 'I lived under apartheid but I didn't support it.' I think we are looking in some communities at a form of denialism down the road. Denialism is a big danger in the future."

White identity in post-apartheid South Africa is the subject of books, public debates and cartoons such as Jonathan Shapiro's Whites who never benefitted from apartheid – a blank space. The political satirist, who works under the name Zapiro, said: "It evoked many vindictive and nasty responses from white people. I think it scratched a wound that is still open. There are some white people who still don't quite understand how brutal a transition we could have had, and how brutal a transition other countries have had."

Zapiro welcomed the T-shirt initiative. "Every white person, no matter how committed to the anti-apartheid struggle, benefited from apartheid," he said. "Anything that shows some people are aware of how much we benefited would be good."