Fossil fuel protesters occupied Didcot power station early yesterday morning, forcing it to switch electricity generation from coal power to gas as a safety precaution.

The protesters, who range in age from 19-50 and belong to no named group, said they planned to occupy the Oxfordshire station for up to a week and could escalate their action by occupying the inside of one of the flues.

Nine protesters were still 200m up the chimney tonight after entering the power station grounds before dawn, but police removed 13 who had locked themselves to a coal conveyer belt. "It is expected that the remaining protesters in the chimney area may remain overnight until it is safe for them to be bought down in daylight," said Npower in a statement.

"We have barricaded ourselves into a round room through which the four flues run. There's no way we can be reached, we're in control of this power plant and we're not moving any time," said Amy Johnson, one of the protesters.

Leon Flaxman, a spokesperson for RWE-npower, the German coal company which runs the power station, said its ability to generate electricity has not been affected. "Only three of the station's four units were working at the time of entry, and one tripped for reasons unrelated to the protest. That is now back up and running. They are all running on gas but we could switch back to coal. We have an alternative way of getting coal [to the boilers]."

According to the protesters, only a small amount of coal was in the boilers when they entered Didcot at 4am, and will last for a few hours unless police clear the 13 protesters, who have locked themselves to the coal conveyer belt. If the company then has to then shut any of its boilers, it will be possible for protesters to climb into its flue pipe, so preventing it being started up again.

"It's an option. But if we went in [to a flue] now it would kill us," said Johnson, an Oxford university student. "Climate change is already killing 300,000 people a year, but we have no intention of dying," she said.

A report in May by Kofi Annan's thinktank, the Global Humanitarian Forum, examined the impact of climate change on humans and found that it is responsible for the deaths of 300,000 people every year.

Flaxman warned the protestors should "absolutely not" enter the flues. "It would be lethal. It takes at least a week after the boilers shut down before people can enter them", he said.

The protesters, who met at a Climate Camp in London this year, said they had targeted Didcot because RWE-npower is planning to build as many as 30 new coal-fired power stations across Europe, including two in Britain. "They have been hiding behind the skirts of power company E.ON and its [proposed new station at] Kingsnorth. This became a symbol of new coal in Britain, but npower is actually the foremost advocate for new coal. We're saying to them that we won't leave until they cancel all their plans for new coal," said Johnstone.

RWE-npower responded that it was only planning one new coal power station in Britain, at Tilbury, and planned to reduce the company's carbon intensity by one-third by 2015. "Didcot is closing down in a few years anyway. We are planning to spend £1bn a year for 10 years on cleaner power, including carbon capture and storage and wind farms," said Flaxman.

The siege of Didcot is particularly embarrassing for RWE-npower because the plant has just upgraded its security. "To be honest we're quite surprised at how easy it all was. I didn't quite expect to be here," said one of the protesters.

Didcot has been the scene of long-running local protests over RWE-npower's plans to dump fly ash from the power station at Radley lakes, a beauty spot which it owned close to the power station. The plans have now been shelved.