Thirty Greenpeace protesters invaded Didcot's coal-fired power station this morning in a bid to shut it down.

Fifteen environmental campaigners chained themselves to a conveyor belt carrying coal into Didcot station A while about 15 others climbed one of the 200m-high chimneys.

Some of the protesters could be seen waving from the top of the chimney, and it is understood that they were planning to spraypaint a slogan saying "Blair's Legacy" from the top of the chimney.

The demonstration coincided with a visit by Prime Minister Tony Blair to businesses in the area, including Diamond Light Source at Chilton.

The protesters broke into Didcot - which they claim is second most polluting power station in Britain - at 5.30am by walking along an internal rail line.

Police officers arrived at the site shortly afterwards to negotiate with Greenpeace over when the demonstration would end.

Greenpeace campaigns director Blake Lee-Harwood, 44, told the Oxford Mail while chained to the conveyor belt: "It was a dirty walk in the dark for us this morning but it was worth it.

"We have all chained ourselves to this conveyor belt so if (plant owners) nPower switch it back on something very nasty will happen to us.

"We are going to hold out for as long as we possibly can because Didcot A can burn gas or coal.

"The longer we stay here, the longer they can't use coal and then they will be forced to switch to gas, which is less polluting."

Police spokesman Kate Smith said: "Officers have carried out a health and safety assessment, and the main concern is the health and safety of both officers and protesters.

"The team of officers includes specialist negotiators and experienced climbers in a rope access team.

"It doesn't look like there is going to be a quick solution to this - it will go on for at least several hours because of the health and safety risks."

Miss Smith added that a command centre has been set up at Abingdon police station and that one man has been arrested for aggravated trespass.

Kelly Brown, a spokesman for nPower, which runs Didcot power station, said: "At the moment the power station can continue to operate, but if safety is compromised we will have to review that. We need this to be resolved sooner rather than later."

The company is expected to review security following the break-in.

Ben Stewart, 32, one of the Greenpeace protesters who climbed the chimney, said protesters had enough food and water to stay for several days.

He added: "It was a steep climb - 1,052 steps and we now have a brilliant view of the millions of tonnes of coal below that is causing unnecessary pollution.

"We have got lots of warm clothing and we are prepared to stay here for as long as it takes."

Laura Yates, Greenpeace spokesman, added: "Tony Blair has had 10 years in power and all he has produced is hot air.

"He has talked and talked about climate change and done nothing. It's incredible that this power station can exist given the amount of talk about climate change."