A coalition of local residents and environmentalists marched from Hatton Cross underground station in west London to the village of Sipson, where some 700 houses will be razed if the expansion goes ahead.
'A stream of people have been arriving since this morning,' Tamsin Omond, a member of the 'Plane Stupid' group said. The Metropolitan Police estimated 3,000 were at the rally.
In February, the 'Plane Stupid' group scaled the roof of Westminster to mark the end of public consultation on a third runway. It has threatened further direct action against government plans to expand Heathrow.
Prior to the start of the 'Make a Noise Carnival', letters of support from Archbishop of Canterbury Rowan Williams and London Mayor Boris Johnson were read out opposing the expansion.
Campaigners will spell out a giant human 'No' which will be visible from the air as MPs, local council leaders and environmentalists address the rally.
Britain's government, business leaders and airport operator BAA say the world's busiest international airport must expand or lose out to continental rivals, damaging the country's economy.
But campaigners say expansion of British aviation makes a mockery of government plans to cut carbon emissions.
The public consultation over the expansion ended in February. The British government is expected to make a final decision later this summer.
