NANTES, France (Reuters) - French riot police clashed with protesters for a fifth day on Friday as their operation to clear squatters from an abandoned airport site evolved into a violent standoff that looked like it could go on for some time.

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After a dawn swoop on Monday to demolish makeshift shelters housing eco-activists and anarchists, police have been engaged in running skirmishes with those refusing to leave and said hundreds of “ultra-leftists” had arrived intent on violence.

The local prefect said a police helicopter had been targeted by someone firing birdshot and police described one attack on their ranks as an “ambush”.

Dozens of people have been injured and several arrested, with the Rennes prosecutor saying five people were in custody.

The continuing violence in the Notre-Dame-des-Landes region, near the city of Nantes in western France, is a challenge for President Emmanuel Macron who also faces student sit-ins as he tries to push through education and economic reforms.

In Paris, police were called in to evict 200 protesters who had occupied part of the Sorbonne university overnight.

“There will be no lawless zones in France,” Christophe Castaner, minister for parliamentary relations and a close Macron ally, told BFM TV.

In Notre-Dame-des-Landes, police were struggling to hunt down mobile squads of protesters who were able to strike and escape easily into a vast area of moorland.

The head of the national gendarmerie, Richard Lizurey, said scores of troublemakers had arrived in recent days, lifting the number facing police to about 700 from 150 at the outset.

“These are ultra-leftists who’ve come here for nothing other than a fight,” Lizurey told CNews TV as heavily armored police fanned out across a densely wooded area.

“We will stay here as long as we have to,” Lizurey said, adding that 10 police had been ambushed on Thursday.

Police say they have come under fire by jars of excrement flung from catapults.

The activists, known as “Zadists” due to their occupation of what they call a ZAD or “zone à défendre” (“zone to defend”) said there had been several injuries from police grenades.