Five climate change protesters have been arrested on suspicion of aggravated trespass at the site of a gas-fired power station.
Nottinghamshire Police said some of those involved in the protest had scaled buildings at West Burton Power Station after gaining access to the site at about 1.20am.
A police spokesman said: "Around 10 (protesters) are thought to have climbed the water towers and have secured themselves to restrict their removal. Searches are ongoing to find a number of other people who are also believed to have gained access to the site."
One of the protesters told the Press Association a total of 17 people had climbed two towers roughly 80 metres apart. Speaking by mobile phone, the woman, who gave her name as Ewa, confirmed the group was demonstrating against climate change and fuel poverty, as well as to highlight the need for renewable energy rather than "expensive and dirty" gas.
Steve Pryle, from the GMB union, said: "All our members have been sent home. We were told approximately 200 of our members have been sent home." He said he had been told there were 26 protesters on site.
The "no dash for gas" campaigners are protesting against moves to build new gas-fired power stations such as West Burton, warning it would leave the UK dependent on a polluting and increasingly expensive fossil fuel for decades.
The Government has backed the development of up to 20 gas-fired power stations as part of its strategy for future energy supplies. Ministers insist that continuing use of gas without technology to trap and permanently store carbon emissions is compatible with targets to cut greenhouse gases to tackle climate change.
It is believed more than 20 people made their way into the power plant in the early hours of the morning, with around a dozen climbing two chimneys at the site, a Nottinghamshire Police spokesman said. Negotiations have begun to encourage the protesters to end the demonstration.
Five of the protesters, all women from either Manchester or Leeds, were arrested as the group made their way into the site at around 1.20am on Monday, he said. They are being questioned at Nottingham police station, he added.
Assistant Chief Constable Paul Broadbent said: "People have the right to protest, but not when it prevents, or restricts the rights of, others from going about their lawful business. There are 101 ways to protest lawfully. Breaking into someone else's property is not one of them. Those involved in planning and participating in this illegal action can expect to be arrested."