Police arrested 28 people today as demonstrators marched on Britain's biggest coal-fired power station in a bid to shut it down.
Chief Inspector Barry Smith said the majority were arrested outside Drax power station in North Yorkshire, but conceded that "a small number" had gained access to the plant.
He said the protesters were arrested for offences ranging from possession of offensive weapons to causing a public nuisance and criminal damage.
The demonstrators set up camp at the weekend about two miles from Drax and planned a day of "creative mass action" for today to close down the huge site, which they say is the UK's biggest single emitter of carbon dioxide.
Mr Smith said: "There was no disruption whatsoever to the functioning of the power station at all.
"There are no protesters inside the power station itself. There are a number on land adjacent to the power station."
The officer said seven forces had helped to staff the police operation and he warned that criminality of any type would not be tolerated.
He told reporters gathered close to the power station that a number of protesters had managed to climb a lighting tower this morning, but had since been taken down and arrested.
He said there were fewer demonstrators than the force had anticipated. Officers had been told there would be around 2,000 people, but estimated that around 600 were there today.
He said at least two of the 28 people in custody were arrested on suspicion of carrying offensive weapons.
This morning hundreds of demonstrators left the site close to the village of Barlow, led by a group of small children and parents pushing pushchairs.
As they made their way up the small lane to the main road, they were joined by another large group of demonstrators dressed in white overalls and with their faces covered.
As they reached the top of the lane where dozens of police vans were parked, a single line of police blocked their way, apparently trying to prevent them from entering the village of Barlow.
A large group then moved off down a farm track in the opposite direction while a smaller group remonstrated with the officers.
There were a few minor scuffles as police attempted to stop demonstrators skirting around their line.
Once the bulk of protesters had moved off across the field, apparently in the opposite direction from the giant power station, about 50 campers remained dancing, playing loud music and chatting.
One of the demonstrators, Alex Harvey, said about 900 people had passed through the site during the week and there were about 800 people there today.
She said a group of about six protesters breached the gates of the power station during the night and had climbed a lighting tower.
North Yorkshire Police have brought in reinforcements from six other forces from across Britain for a major security operation to protect the power station, which is located between Selby and Goole.
The force's Chief Constable, Della Cannings, arrived at the power station this morning to assess the situation and to check everything was running smoothly.
"We want to minimise disruption to the community and prevent criminal offences taking place," she said.
"We will take quick action if necessary."
Ms Cannings said she was pleased there had been no increase in criminal activity in the local community whilst the camp had been in place.
Police roadblocks were set up along all the main routes to the plant, causing long tailbacks for commuters.
Dozens of police vans have been strategically placed around the area.
In one layby, 10 vans from the Metropolitan Police were seen parked up, and on the main Selby Road 25 police Transit vans were spotted in convoy.
Fencing has also been put up across the railway lines skirting the plant.
A North Yorkshire Police spokesman said officers were in place in an attempt to "facilitate any peaceful demonstrations and deal firmly with anyone who steps outside the law".