Some 8,000 people, according to police figures, took part in a festival at Notre Dame des Landes on the day after its official designation as a site for the planned airport ran out.
French environmental activists were joined by supporters from Spain, Italy, Germany and Belgium and politicians, including MEP José Bové.
The government decided to ditch the project in January, 45 years after it was first proposed.
Festival participants vowed to take their fight to other controversial projects, such as a proposed high-speed rail line between Lyon in France and Turin in northern Italy and a planned nuclear waste dump at Bure in the east of France.
In nearby Bouguenais, about 300 residents of the area around the already existing Nantes airport threw their electoral registration cards into a coffin in protest at the government's decision to ignore the results of a local referendum that showed 55 percent of participants in favour of building a new one.
Dispute over land begins
The government now has to decided what to do with the 1,650 hectares of land that have been occupied by farmers and environmentalists opposed to the project.
Its representative in the region, Nicole Klein, has promised a "peaceful and constructive consultation" but battle-lines have already been drawn up between the local chamber of agriculture, dominated by conservative farmers' unions, and the radicals who took over the site.
The protesters want control to be handed over to a new body, such as the one that runs land on the Plateau de Larzac, following a similar occupation that prevented it being taken over by the military in the 1970s.
Bové, who took part in that campaign, supported that choice on Saturday, saying that the number of farmers at the Larzac site had grown by 26 percent since its establishment.
Chamber of agriculture president Jacques Lemaître says the protesters "have no business being there" and argues that his members have expertise and experience.
Owners who sold their land have the right to buy it back at the price they were paid and the 30-odd farmers who refused to sell have already started legal proceedings to have their rights to their property restored.
Last month, when he announced the end of the airport project, Prime Minister Edouard Philippe gave "illegal occupants" until the spring to "leave of their own accord or they will be ejected".