A week-long climate change protest near Heathrow airport is continuing amid signs of growing tension between campaigners and police.
Policing levels have now returned to normal at the camp
An attempt to increase police numbers has been blocked and a woman arrested after gluing herself to a gate.
Scotland Yard said its four officers at the camp had met with opposition and more staff had been sent in as support.
The protesters want Heathrow's planned expansion halted because they say it will contribute to climate change.
Organisers said more than 100 demonstrators prevented about 30 officers from entering the camp overnight.
Timothy Lever, a spokesman for the camp, said: "A large number of police attempted to break into the camp and they were peacefully removed by a large crowd of protesters with their hands in the air who gradually moved the police away.
"The police gave us no warning and did not say why they were coming on the site. They did it after the media left."
Scotland Yard said a uniformed forward intelligence team met opposition from some of the members of the camp, and more officers were sent in support.
A statement said: "Police worked with the camp liaison to resolve the issue. Policing levels have now returned to the normal patrolling officers."
Protester Penny Eastwood, of Hebden Bridge, West Yorkshire, was arrested on suspicion of criminal damage after she glued her hand to a metal gate on the edge of the camp.
Organisers say 550 people have now joined the camp - between the M4 motorway and the airport's northern perimeter in west London - and expect up to 2,000 people to take part in the protest this weekend.
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 an High Court injunction banning certain protest groups from the airport.
Organisers of the protest 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.
But 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 so 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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