Hundreds of climate change protesters clashed with police as they tried to break into a power station in a bid to close it down.
Officers said they were tackling 'concerted disorder' at Ratcliffe-on-Soar power station, near Nottingham, despite arresting 10 protesters this morning before the trouble started.
Activists from three pressure groups - the Camp for Climate Action, Plane Stupid and Climate Rush - staged the protest as part of a campaign against fossil fuel plants. Ratcliffe is one of the biggest coal-fired power stations in the country.
Police have been trying to block protesters who have carried out threats to tear down fencing in an attempt to close down the site.
At 3pm a spokeswoman for Nottinghamshire Police said: 'Police officers stationed at the perimeter of Ratcliffe-on-Soar Power Station are facing groups of protesters, around 50-strong in places, trying to gain entry to the site.
'Officers are using appropriate force to stop the protesters, but are not trying to contain them. All protesters are free to move away from the perimeter.
'A breach of the fence line has now been secured and a number of further arrests have been made.'
Reinforcements were called to support the police response.
The spokeswoman said: 'We've got a significant police presence which is increasing as the situation develops.
'We are facing concerted disorder and we will continue to try and facilitate peaceful protest.'
Earlier the force arrested seven men and three women on suspicion of conspiracy to commit aggravated trespass at the power station.
Nine of the arrested were from Manchester and one came from West Yorkshire.
Camp for Climate Action said the arrests were "outrageous".
The 2,000MW power station, owned by the energy company E.ON, is one of the largest producers of carbon dioxide in Britain.
The activists, under the banner the Great Climate Swoop, chose to target Ratcliffe in an online vote following the Climate Camp held in London in August.
They want all coal-powered power stations decommissioned.
Around 500 protesters were at the site this morning.
E.ON was granted a High Court injunction giving police the power to arrest anyone trying to enter the site.
A company spokesman said: 'Ratcliffe power station plays a key role in ensuring we keep the lights on for millions of homes across the region and has undergone major investments that make Ratcliffe one of the cleanest power stations of its kind.'We're already closing three of our other fossil-fired power stations and are pushing forward with lower carbon options that include developing new wind farms, gas plants, cleaner coal and nuclear that will allow us to tackle the threat of climate change while also ensuring security and affordability of energy supplies.'
The demonstration is expected to start at 1pm.
A 21-year-old man was arrested on Wednesday on suspicion of conspiring to damage property at the power station.
He has been released on bail pending further enquiries and must return to his local police station in Leeds on Saturday.
Staff have been sleeping at the Ratcliffe plant over the weekend to ensure continuing power for homes.
In April 114 people were arrested at a Nottingham school on suspicion of planning direct action at the power station.
Charges of conspiracy to commit aggravated trespass were passed against 25, who face restrictions on communications and possible sentences of six months.
Activists have also targeted E.ON's plans for a new coal-fired power station at Kingsnorth in Kent. Last week the company said the plans had been postponed because of the global recession.