Police Chief Superintendent Ian Thomas said 25 officers inspected the Camp for Climate Action site at the request of the landowner, Imperial College London.
Thomas said organizers had agreed to allow four officers to remain on the site for the duration of the week-long demonstration.
Organizers of the event predict as many as 2,000 people from Britain and elsewhere will converge on the site to protest the addition of another runway.
The group claims air travel is a major factor in greenhouse gas emissions, the BBC reported.
Spokeswoman Gemma Davis told the network the intention was not to delay air travelers, but rather disrupt the British Aviation Authority, which oversees airport operations.
The group said in a statement the demonstration would conclude on Sunday with a day of "mass direct action," the report said.