The protests at Heathrow Airport have been infiltrated by troublemakers who are "quite happy to break the law", a senior police officer has said.
Police officers have been on patrol at the protest camp
Assistant Commissioner Tarique Ghaffur said many of them were looking for an opportunity to confront the police.
But protest organisers said the Met officer's comments were "ludicrous" and "part of a smear campaign".
Meanwhile, 10 people have been arrested after chaining themselves to the gates of Biggin Hill Airport, south London.
The protesters at Heathrow want its planned expansion halted because, they say, it will contribute to climate change.
Growing tension between campaigners and police at the Camp for Climate Action has seen attempts to increase police numbers blocked.
Mr Ghaffur said extremists may "suck in" normally peaceful protesters.
He said: "While we do know they have environmental sympathies, we know that some of them are quite happy to have confrontation with the police and are quite happy to break the law."
Mr Ghaffur added that camp organisers had shown "very little co-operation" towards police.
Commander Jo Kaye, also from the Metropolitan Police, said the mood at the camp had worsened since the troublemakers had turned up.
He said: "If we frustrate them then they will go in for confrontation because their aim is anti-state.
"The environment is part of that, but we are talking about anarchists, so the cause assists their overall cause."
But Paul Sumburn, a spokesman for the camp, said: "This is part of a concerted smear campaign against us.
"There was no talk of confrontation in our action discussion.
"The idea that the camp has been infiltrated is ludicrous. It's funny that these infiltrators only talk to police and undercover journalists."
About 1,800 officers are to be deployed on Sunday and Monday, when the main demonstrations are planned.
So far there have been 21 arrests.
Organisers expect up to 2,000 people to take part in the protest at the camp this weekend, between the M4 motorway and the airport's northern perimeter in west London.
About 1,800 officers from Surrey police, Thames Valley police, the Metropolitan Police, and British Transport Police will oversee the event.
Airport operator BAA has obtained a High Court injunction banning certain protest groups from the airport
Protestors say the first few days will be taken up with 100 workshops on issues ranging from campaigning skills to practical training on how to take direct action.
However, a website supporting the camp has also promised acts of "civil disobedience".
Police say the protesters are on the site - a sports ground belonging to Imperial College London - illegally.
The campaigners insist they cannot be evicted without a court order as long as no criminal damage is committed.
A fifth terminal will open at Heathrow in March 2008 and a new runway has been proposed by the government for about 2020.
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