Four people have been arrested during at protest by 600 environmentalists at the Drax plant in northern England.
The Camp for Climate Action group which organised the event said nine protesters were also inside Britain's biggest power station
The group's aim is to invade the station and force it to close.
Drax Power, which generates about 7 per cent of the Britain's electricity, has hired extra security staff and police have been drafted into the area.
Small scuffles have broken out between police and protesters.
Coal-fired Drax, at Selby in Yorkshire, is the United Kingdom's biggest single industrial emitter of carbon dioxide, the main greenhouse linked to global warming and climate change.
Drax's carbon dioxide emissions in 2005, at 20.8 million tonnes, were higher for example than Sweden's 19.3 million tonnes.
David Porter, chief executive of the Association of Electricity Producers, condemned the group's actions, saying every electricity generating business in Britain was taking steps to address climate change.