Eco-activists who scaled a 650ft chimney at Didcot Power Station woke up a bright frosty morning today in their bid to bring the plant to its knees.

Greenpeace campaigners scaled the tower in the early hours of yesterday morning and a second group chained themselves to the station's coal feed.

The stunt had an impact on electricity output but station bosses insist it will not affect customer supply.

Two teams of protesters broke the station's security cordon at around 5.30am in what was clearly a carefully orchestrated operation.

Under the cover of darkness, a group of around 10 activists headed for the station's chimney stack, looming 650ft above the ground, and climbed to its summit.

A second team of 15 switched off the plant's coal feed before chaining themselves to the coal conveyer belt 60ft above the ground.

The activists said they had encountered little resistance but had identified themselves to security staff as members of Greenpeace.

Around 50 police officers, including specialist negotiators, experienced climbers, public order officers and a mounted section, were quickly on the scene heralding the start of a long, and ongoing, stand-off.

A special command centre was also established at Abingdon police station. At around 10am, two protesters abseiled down the tower painting the slogan 'Blair's Legacy' in large white letters.

Protesters on the coal conveyor belt also unfurled a banner which read 'Two Thirds Wasted' - a reference to Greenpeace's claims that the plant is inefficient.

By lunchtime power output had been reduced in a bid to conserve dwindling coal supplies for the evening peak demand.

Despite ongoing negotiations between the police and a Greenpeace team on the ground there was no sign of a resolution by nightfall.

Overnight temperatures were set to fall to -3C.

Greenpeace campaigner Laura Yates, who was stationed outside the plant's perimeter fence, said: "Our aim is to stop power generation and shut the plant down until we have a commitment from Government that they will not allow another power station like this to be built."

Ben Stewart, one of the protesters at the top of the chimney, told the Oxford Mail they had no plans to come down.

He said they had food, water and clothing to remain in place for three to four days. He added: "We're in good spirits and we're bedding in for the long haul."

Police superintendent Jill Simpson, area commander for south Oxfordshire, said negotiations would continue and the safety of protesters and officers was paramount to the operation.

She said removing protesters by force had been deemed a risky strategy but added all options, including force, were being assessed.

But Supt Simpson admitted it was likely to be a long process.

Last night, Didcot A Power Station manager John Rainford said owner nPower wanted a quick, safe and peaceful end to the protest and added the stunt had financial implications for the company.

He said there was enough coal within the turbine hall to last several hours but one of the two operational turbines could be switched to gas, and bio-fuel use increased, to help alleviate the situation.

Mr Rainford said production at other power stations could also be increased to meet demand.

He said security at the station would be reviewed in light of the protest.

Speaking exclusively to the Oxford Mail, Prime Minister Tony Blair said: "I think people that do this are being silly because shut Didcot power station and you shut down power for thousands of people."

One man has been arrested on suspicion of aggravated trespass.

The invaders

Greenpeace is a non-profit organisation, with a presence in 40 countries, and claims to have 2.8 million supporters.

Since 1971, it has campaigned to stop climate change, protect ancient forests, save the oceans, stop whaling, eliminate toxic chemicals and encourage sustainable trade.

The Rainbow Warrior is the name Greenpeace gave to a number of its ships. The first one was sunk by frogmen working for French intelligence in New Zealand in 1985. The ship was to lead a flotilla of yachts protesting against French nuclear testing at Mururoa Atoll in French Polynesia when it was sabotaged.

Of the 12 people on board, photographer Fernando Pereira drowned when he attempted to retrieve his equipment.

The power station

The npower site at Didcot has two power stations. Didcot A - which was targeted by Greenpeace - began operating in 1970 as a coal-fired power station.

It can generate enough electricity to meet the needs of two million people, and three of its generating units have been converted so that they can burn natural gas as an alternative to coal.

Didcot B is one of a new generation of highly efficient, gas-burning stations. It is powered by natural gas and uses the latest technology to produce electricity in a more environmentally-friendly way.

A £3.5m bio-fuel plant launched last month will allow the station to use carbon neutral bio-fuel - cutting its annual carbon dioxide emissions by 700,000 tonnes.

A focus for protest

It is the third time this year that environmental protesters have targeted Didcot power station.

In July, activists carried out a dawn raid similar to the one yesterday but on a much smaller scale. They climbed the perimeter fence at dawn before climbing a 100ft lighting tower. Five protesters unveiled a 50ft banner with the words "Climate Crime" across it. The stunt was organised by the newly-formed Reclaim Power organisation - set up to take peaceful, direct action against climate change.

The same month, Save Radley Lakes campaigners turned a cooling tower at the power station into a 360ft protest, by beaming a slogan on to it ahead of a meeting with county councillors. Station owner RWE npower plans to dump spent fuel ash into Thrupp Lake in Radley.