Hundreds more people are expected to join protesters already camped next to Heathrow Airport to demonstrate against global warming.
About 1,800 officers will oversee the protest next to Heathrow Airport
The week-long protest will call on the government to halt Heathrow's planned expansion, which campaigners say will contribute to climate change.
Protesters denied reports they planned to issue bomb hoaxes and disrupt the terminals disguised as passengers.
Downing Street has said any disruption to the airport would be "unacceptable".
A fifth terminal is to open at Heathrow in March 2008 and a new runway has been proposed by the government for about 2020.
Protesters from the UK and abroad have been gathering at the Camp for Climate Action since Sunday to highlight their claims that the growth in air travel is a major factor in greenhouse gas emissions.
Up to 250 people are already at the camp between the M4 motorway and the airport's northern perimeter.
Organisers say they are expecting up to 2,000 people to join the protest this weekend.
About 1,800 officers from Surrey police, Thames Valley police, the Met, and British Transport Police will oversee the event.
Commander Jo Kaye, who is responsible for the operation said policing the protest would not affect day-to-day counter terrorist operations at Heathrow.
Mark Bullock, the managing director of BAA, Heathrow's operator, said he was "extremely concerned" at reports that protestors planned to disrupt the airport disguised as passengers and issue hoax bomb threats.
But camp spokeswoman Sophie Stephens insisted that activists intended to target the aviation industry itself, not passengers.
She added: "These reports should be given no credence at all.
"We've made it clear that the protests will be non-violent and we will not be going onto the runway."
Emily Armistead, a senior transport campaigner at Greenpeace, told BBC Radio's Five Live that action was being taken at Heathrow because the government had "completely ignored the role of aviation".
"Aviation emissions aren't even part of our climate budget - they don't fall within Kyoto - and for that reason the government has just given the aviation industry a green light to expand when the rest of us are being told we have to reduce our emissions," she said.
But Anthony Concil, from the International Air Transport Association, told Radio Five Live aviation was being unfairly blamed for the "global problem" of climate change.
"We're 2% of the problem - and that's not to excuse us from trying to reduce that number," he said.
"Airlines are working in a number of ways to try and do that, with governments, with air traffic control, with airport infrastructure operators, but clearly the attention that's being given to the industry is disproportionate to its contribution."
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".
Chief Superintendent Ian Thomas said the protesters were on the site - a sports ground belonging to Imperial College London - illegally.
A spokeswoman for the college said it had not given permission for the protest and hoped it would be "peaceful, safe and not damage the land".
"Any person who gains access to and/or occupies these fields is doing so unlawfully and should vacate the field," she added.
But the protesters insisted the law was on their side, and that they could not be evicted without a court order so long as no criminal damage was committed.
Last week, BAA won a High Court ruling banning certain protest groups from the airport.
Domestic aviation contributes less than 1% of Britain's carbon dioxide emissions, but the CO2 from all international flights leaving the UK amounts to nearly 7% of the national total.
Aeroplanes also release other greenhouse gases including ozone, and affect cloud formation. This means their overall impact on global warming is about three times larger than their production of CO2.
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