Using around 10 million tonnes of coal a year, Drax produces 4,000 megawatts of electricity, or about seven percent of Britain's total.

Drax says it also churned out 20.8 million tonnes of carbon dioxide last year, which The Times newspaper reports is more than that produced by 103 nations and compares to 91 million tonnes of carbon dioxide per year produced by vehicles in Britain.

Police said four people were found inside the Drax perimeter on Thursday and were arrested on suspicion of trespass. Two others had been arrested overnight on suspicion of carrying offensive weapons.

They said there were about 600 protesters at Drax and they had drafted in reinforcements from other forces to protect the power station.

Chief Constable Della Cannings said: "We want to minimise disruption to the community and prevent criminal offences taking place. We will take quick action if necessary."

Demonstrators, some dressed in white overalls and with their faces covered, marched from their camp and there were minor scuffles with police who blocked them from entering a nearby village.

One protester, Alex Harvey, said about 900 people had passed through the site during the week and there were about 800 people there on Thursday.

She would not say what they planned to do if they got to the power station.

It has 12 giant cooling towers and its 850-foot (259-metre) chimney is the highest structure in Britain barring broadcasting masts.

The station is regarded as the cleanest and most efficient coal-fired power station in Britain, removing 90 percent of sulphur dioxide emissions, the pollutant responsible for so-called acid rain.

However, millions of tonnes of carbon dioxide are produced due to the plant's giant size.

Coal currently accounts for about 34 percent of British power generation, with gas accounting for about 37 percent, nuclear about 21 percent and renewables four percent.

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