Kent Police has received 29 complaints against its officers
Nearly £6m was spent policing the week-long Climate Camp near Kingsnorth Power Station, according to Kent Police estimates.
A Freedom of Information (FOI) request by BBC South East revealed nearly £5.9m was spent on officers, accommodation, air support and planning.
The force said it would be some time before final costs were known.
Climate Camp spokesman Kevin Smith said the cost was an outrageous waste of "public money".
He added: "It's money that has been spent protecting E.On's profit margins, clamping down on people's civil liberties and trying to prevent a much needed public debate on coal and climate change from taking place.
"The sums of money involved show the extent of the government's determination to push ahead with the new power station, regardless of its devastating climate impacts and regardless of the fact that it is so vehemently opposed across the UK."
FIGURES RELEASED BY POLICE
Total cost - £5.9m
Officer overtime - £628,262
Accommodation costs - £359,326
Arrests made - 100
Complaints received - 29
The Chief Constable of Kent Police is currently in talks with the Home Office about its contribution.
The force initially estimated that about 1,400 officers policed the event and 1,000 activists attended the camp, which was held in protest against E.On's plans to build new coal-fired units.
It now says the exact figure has yet to be calculated.
In reply to the BBC's FOI request Kent Police said current officer overtime claims for the climate camp stood at £628,262.
It said the amount spent on accommodation came to £359,326. The majority was used to put up non-Kent officers but the figure also included media and conference facilities.
The force said the cost of using the police helicopter, which made 63 flights during the event, was about £13,000 above the "usual operational requirements".
A breakdown of the remaining £5m estimated costs was not supplied by Kent Police.
Sittingbourne and Sheppey MP Derek Wyatt said: "We asked the police to maintain the security around there.
"What could have happened is that they could have taken over (Kingsnorth). Then you'd have been saying it was justified. Now maybe good policing stopped that."
The figures also revealed that 29 complaints were made against officers.
A number of protesters complained they were ill-treated by police and accused them of heavy-handedness.
Kent Police said it would investigate all complaints about its officers.
Officers made 100 arrests during the event on the Hoo peninsula, about two miles from the power station, in August.
Out of those, 46 were charged, mostly with public order offences or obstructing the police.
Twenty-two were cautioned, three were bound over to keep the peace, and one breached bail terms, the force said.
A spokesman for Kent Police added: "The current, estimated cost stands at around £5.9m.
"However, it will be some time before the final costs arising from the operation are known as invoices are yet to be received from some suppliers and, for example, overtime claims for all the officers involved will take some time to resolved.
"Further, there are other costs to consider such as ongoing legal costs from any litigation or claims that may take some time to reach a conclusion."
The current Kingsnorth power station is due to close in 2015 and E.On wants to replace it with two new coal units, which it claims will be 20% cleaner.
Its proposals have already been approved by Medway Council but the government will make the final decision about the project.
Activists have vowed to return to the power plant if the decision is approved.