Nine Greenpeace activists were arrested at the Kusile power station's construction site in Mpumalanga on Monday, police said.
Seven were arrested for scaling a crane on the site, while two were arrested for chaining themselves to the gate, Senior Superintendent Vish Naidoo said.
He said the seven men and two women were a mix of South African, Dutch and German nationals.
Greenpeace spokesman Mbong Akiy said the activists stood on the crane for more than five hours before being arrested.
They hung banners reading “Kuyisile: Climate Killer” on the 150-metre crane.
A group of about 20 activists demonstrated at the gates of the new power station, in Emalahleni, earlier in the day.
They were protesting against South Africa's reliance on the burning of coal, which emits greenhouse gases, to generate electricity.
Greenpeace claims in a recent document that Kusile's external and hidden costs could be between R31.2 billion and R60.6bn a year.
The Greenpeace-commissioned “True Cost of Coal” report was compiled by the University of Pretoria and was released at the end of October.
It investigated the actual costs of Kusile's entire coal chain, from climate change to water use, the impact on health and the damaging effects of coal mining.
“If Kusile was a country, it would be the fifth most polluting country in the world. It's not sustainable. The impact (of R60bn) on South Africans would be devastating,” Musana said.
“Eskom is reluctant to speak with us... we want to hand over a memorandum with the report findings. We want a just transition from coal to renewable energy as it's a win-win situation in terms of job creation, the climate and energy.”
Eskom earlier welcomed a protest over cleaner energy as long as it was peaceful and did not disrupt operations.
“An important part though is that we don't apologise for building two large coal-fired stations. We are a developing country with a great need for economic growth and job creation,” said spokeswoman Hilary Joffe.
“We need a secure and affordable supply of electricity in the short and long term, and coal is part of that future. We need to balance that supply with reducing emissions,” she said.
Kusile is expected to add 4800 megawatts to the power grid on completion in 2017.
The Medupi power station, being built in Lephalale, Limpopo, will add another 4800 megawatts of coal-powered energy.
The stations would use advanced technology to burn less coal for the same amount of energy and achieve reduced emissions.
Air quality would be improved by removing oxides of sulphur from exhaust gases released into the atmosphere. - Sapa