Scores of protesters are arriving for what organisers hope will be the biggest-ever demonstration against the further expansion of London Heathrow Airport.
Support for the event - being called Make A Noise Carnival - is expected from all over the UK.
The demonstrators will walk from Hatton Cross to Sipson, the village that will be lost if plans for a third runway at Heathrow go ahead.
At Sipson, the protesters will gather in a field to form a huge "NO" that should be visible to passing aircraft.
Politicians and environmentalists are due to address the crowds and bands will play.
Protest organiser Tamsin Omond said: "We are expecting tens of thousands to attend - the weather is good so fingers crossed."
Protesters spoke of the reasons they had made the journey to west London.
Lewis Jones, 21, from Roehampton said: "I'm here just because the public need to know. Economic expansion and growth is all very well for those making the money but the damage it's doing to the planet is not acceptable. This is building momentum."
Camille Barton, 19, from north London is a student and Friends of the Earth supporter. She said: "This protest feels safer than the Climate Camp protest last year. It's been advertised heavily in newspapers, that's how I found out about it. I read an article about Tamsin Omond, the organiser of the protest, and what she's done has been a real inspiration for me."
British Transport Police said the airport eastern and northern perimeter roads would be closed during the march and also a lane on the A4 eastbound will be closed.
