Police said about 1,000 demonstrators had gathered near Heathrow airport earlier in the day.
BAA said the airport was operating normally and there had been no disruption to passengers.
There were minor scuffles as police officers ringed the protesters marching along country lanes surrounding the airport.
Police said they had made four arrests, one for assaulting a police officer, taking to 49 the total number of campaigners detained over the course of the week of protests.
|Campaigners and locals oppose plans to build
a third runway at Heathrow [AFP]
Richard Bestic, reporting for Al Jazeera from London, said: “The big fear on the part of the owners of Heathrow airport, the British Airports Authority, is that there will be people who break on to the runways.”
Several hundred activists have set up a temporary home near the village of Harlington, close to Heathrow, in a tented village called the Camp for Climate Action.
Patrick McDonell, a representative of the Camp for Climate Action, told Al Jazeera that the group was “armed with peer-review science”.
McDonell said the government’s attempts to reduce emissions by 2050 were going to be undermined by its simultaneous plan to expand runways and aviation.”
Asked about reports that protesters had broken the law, McDonell said: “We’re quite prepared to break the law in a non-violent fashion in order to raise awareness.
“There is a long history of civil disobedience from the suffragettes to the civil rights movement.”
The campaigners have joined locals also opposed to plans for a third runway at Heathrow, the world’s busiest international airport handling nearly 70 million passengers a year.
They claim it will exacerbate noise pollution and add to health problems.
BAA has acknowledged that “there is an important debate to be had regarding climate change”.
The activists say their presence for a week of “low-impact living, debates, learning skills, and high-impact direct action” is a peaceful protest.
Last week, protesters had glued themselves to Britain’s department of transport.
According to an airport spokesman, the demonstrations, which are due to end on Tuesday, have not disrupted services at Heathrow.