Thirty-eight campaigners were arrested during a "mass day of action" against carbon emissions at Britain's largest coal-fired power station.
Police said they would deal "swiftly and firmly" with any disorder
Hundreds of demonstrators hoped to disrupt operations at Drax Power Station in North Yorkshire.
Offences included criminal damage, aggravated trespass and possession of offensive weapons.
Police said the majority were arrested outside Drax but said a "small number" had gained access to the plant.
Ch Insp Barry Smith said a giant home-made ostrich had been seized by police, and found to be containing tools which could have been used by demonstrators to chain themselves to fences.
He said North Yorkshire Police were "quite pleased with the way things have gone so far".
He said: "There was no disruption whatsoever to the functioning of the power station."
Mr Smith said there were fewer demonstrators than the force had anticipated. Officers had been told to expect around 2,000 protesters but about 600 were thought to have turned up.
North Yorkshire Police officers and staff from seven other forces were at the power station site and the camp on Thursday "to ensure that residents, power station employees and the demonstrators themselves do not come to harm, and that no laws are broken".
Police roadblocks were set up along all the main routes to the plant, causing long tail-backs for commuters.
Drax is the biggest coal-fired power station in Europe
Dozens of police vans were placed around the area, near Selby, and fencing was put up around the railway lines skirting the plant.
Following the day's first arrests the force's Deputy Chief Constable, Ian McPherson, said: "This is a sad contrast to the sincere and law abiding intentions of the majority"
He said the majority of the protesters were "behaving themselves".
"However we are conscious that within that group there is a hardcore - a number of individuals who have made their aims very clear, see themselves as rising above the law and it's those individuals who I think will undermine what is a serious point."
'Prepared to break law'
Protester Michelle Bernstein, 28, from York, said: "Many of us are prepared to break the law, because the powers-that-be are addicted to planet-trashing economic growth.
"Shutting down a power station isn't enough to stop climate change but it's a start."
DRAX POWER STATION
Commissioned in mid-1970s
Employs 625 people
Supplies 7% of UK's electricity needs
Damaged by fire in 1999
Almost went into administration in 2003 amid UK power market slump
Revived by an unprecedented rise in wholesale electricity prices
Joined the stock market in December 2005
About 600 people were staying at the Camp for Climate Action, next to the power station.
Drax is the biggest coal-fired power station in Europe, more than twice as big as any other power station in the UK.
Protesters claim it is obsolete and should be closed down and replaced with cleaner forms of power.
Drax Power was granted an injunction earlier this month banning unauthorised people from entering the site or using an adjacent footpath.
The injunction, to last indefinitely, was brought because Drax said it did not want protesters to be harmed by the complex industrial processes at the plant.
It also sought the injunction because it said Drax was an "important national strategic asset", contributing 7% of the country's electricity.
Drax Power's chief executive, Dorothy Thompson, said the company was "very concerned" about the impact of climate change but found it hard to understand what the demonstrators hoped to achieve.
She said: "What I don't understand is how these protesters think it's practical to shut all the coal in the UK, all coal generation in the UK, on a very short-term basis.
"What that would actually lead to would be chronic energy shortages in the UK and actually a serious shock to the economy."