Up to 30 police officers were blocked by protesters as they attempted to force their way onto the Heathrow airport climate change camp outside London, campaigners said tonight.
Organisers said a surprise move by police was greeted by alarms and more than 100 demonstrators raced to crowd the police officers back off the site.
The campaigners plan a week of protests in a bid to force the government to halt expansion at the airport.
Scientists say air transport contributes heavily to global warming.
Timothy Lever, one of the camp's spokesmen, 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."
Protesters have fiercely defended the field they are illegally occupying close to the boundary of the west London airport.
Since the camp first appeared on Sunday they have only allowed up to four officers to patrol on site to check for illegal activity.
Tensions rose between protesters and police earlier today as officers attempted to photograph many of those arriving at the camp.
Protesters, who recorded the incident which took place tonight at about 7.30pm, said police claimed afterwards they were attempting to count the numbers of those on the site.
In a statement, Scotland Yard said: "Uniformed forward intelligence team officers patrolling the camp met with some opposition from some of the camp participants. Additional officers were then deployed in support of them. Police worked with the camp liaison to resolve the issue. Policing levels have now returned to the normal patrolling officers."
Earlier in the day, protesters refused to rule out illegal activity as they began to discuss plans for a "day of action" at the airport on Sunday.
But they insisted they would not resort to violence.
Police doubled their numbers at the site, with mounted officers, sniffer dogs and an ANPR (automatic number plate recognition) van working alongside vanloads of officers.
Fears were further raised when camp organisers confirmed they had sent an email urging activists to bring smart clothing to blend in with ordinary workers.
BAA, the airport's operator, said it agreed there was a debate to be had about aviation and climate change.
"But the 1.5 million passengers who will travel through the airport during the (week-long) camp have a right to go about their lawful travel plans without being harassed or intimidated," BAA said in a statement.
Heathrow handles an average of about one flight a minute and a total of nearly 70 million passengers a year.
PA