Five Greenpeace activists were roughed up by South African private security guards and briefly detained during an anti-nuclear protest in Johannesburg on Tuesday, a spokeswoman said.

They were part of a group of around two dozen protesters outside the offices of South Africa's Industrial Development Corporation (IDC), where a conference on nuclear energy in Africa was under way.

"The security guards were very aggressive and very rough in terms of how they dealt with the protest," spokeswoman Ferrial Adam told AFP.

They chained themselves to the gates of the complex but the guards managed to dislodge them and held them in a room for 30 minutes.

She said "no hospitalisation was needed but we were bruised because they used batons on our backs."

"We are protesting against a nuclear conference being held at IDC. We are saying that it is extremely dangerous, it is expensive, it will bankrupt Africa," she said.

"We have enough amounts of clean energy."

South Africa, the only continent's only country to possess a nuclear plant, is planning to expand nuclear capacity as part of a $127-billion scheme to overhaul the national energy supply.