The clashes erupted at dawn when protesters began throwing projectiles at officers from behind barricades set up to defend the camp at Notre-Dames-des-Landes near the city of Nantes.
By midday the protesters' ranks had swollen to some 150 people at the location of the clashes, with police firing tear gas and stun grenades while activists set fire to at least one makeshift barricade and an abandoned car to halt their advance.
A police spokesman told France Info radio that in total about 150-200 people "had come to reinforce" the protesters across the large camp in the wooded area.
Many were equipped with gas masks, molotov cocktails, makeshift shields and racquets they used to knock back tear gas cannisters.
The clashes have escalated since the expulsion operation began Monday with the deployment of about 2,500 officers to the site, 28 of whom have been injured.
According to a medical team set up at the activists' camp, about 20 protesters had been hurt on Wednesday, with one hospitalised, after about 30 were hurt on Tuesday.
An AFP photographer was also lightly wounded in the legs by fragments of a stun grenade.
"If the police do not pull back their troops tonight, we'll call a general mobilisation," said Julien Durand, spokesman for one of the main associations opposed to building the airport.
'We were had'
Activists opposed to plans to build a new airport near the city of Nantes first began squatting on the farmland in 2008, and the camp grew into a sprawling 1,600-hectare (4,000-acre) settlement billed as a utopian leftist farming community.
But the government announced in January that it was calling off plans for the airport and warned the squatters that they must clear off the land by spring.
The three days of running battles echo a failed attempt to clear the camp in 2012.
The activists are furious at police damage to their shelters and farming projects including a sheep shed and cheese-making area, saying they had been in talks with local officials on maintaining many of the projects.
The government had said activists could stay on the land if they came up with individual farming schemes but most refused, saying they want to run the site collectively and be able to pursue non-agricultural projects.
"We played the game, and we were had," said a protester who gave her name as Sarah.
"We are outraged, what else could we be."
Local authorities say 16 of the encampments dotting the farmland were cleared in the first two days of the operation, 15 of them demolished.
The plan is to dismantle up to 40 as authorities seek to retake control of a key road running through the area that has been blocked for five years.
Interior Minister Gerard Collomb has said the operation could last until the end of the week, while the government's top official in the region, Nicole Klein, said not every structure would be razed.
"We have only dealt with the squats made of boards," she said late Tuesday, adding that talks between officials and protesters would resume "as soon as possible".
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