People opposed to a proposed new runway at Heathrow Airport have set up a camp to highlight air travel's contribution to climate change.
Supporters of the week-long Camp for Climate Action have occupied land between the M4 motorway and the airport's northern perimeter.
Up to 2,000 people are expected to attend the camp, due to culminate in a day of "mass direct action" on Sunday.
Up to 1,800 police officers will be on duty during the protest.
Twenty-five police officers inspected the camp on Monday, with four remaining on the site.
Airport operator BAA has warned protesters it will not tolerate any attempts to disrupt passenger flights.
Protest organisers said their aim was not to disrupt people's travel plans, but to start a debate about airport-linked environment damage.
Organisers plan to run workshops for the first few days of the camp, which officially begins on Tuesday.
A fifth terminal is scheduled to open at the airport in March 2008, and the government has proposed a new runway there for about 2020.