Farmers encircling Paris with their tractors may seem good-natured - but their purpose is serious

The first thing you see are the twinkles of blue flashing lights, reflected over the distant brow of a hill.

A minute later, the flash of headlights and then the sound of horns. And finally it all comes into view.

In front of us, a column of tractors is driving down the A4 motorway, one of the main roads that links Paris with the rest of France.

The rush hour has barely finished, but the tractors are the only moving vehicles I can see other than a couple of police cars driving alongside.

The motorway is blocked; only these tractors are moving.

Behind me, the applause and cheers of a big group of farmers who have already taken up position, blocking the road in both directions.

A fire has been lit in the scrubland to the side of the motorway while, just to my right, a flare is billowing orange smoke across the road.

Amid it all, one of the tractor drivers is using his horn to play the tune of Baby Shark.

Bearing in mind the seriousness of these protests, there is an air of celebration among those who have just arrived.

Perhaps they are a little delirious. These tractors have come a long way to get here to Jossigny, on the outskirts of Paris.

They have travelled from the eastern reach of France - from around Moselle, close to the border with Germany.

A journey of more than 200 miles in a vehicle that trundles along at around 25 miles per hour might be enough to turn anyone to Baby Shark.

But their purpose is deadly serious.

"It breaks my heart to have to do this," says Antoine, as he climbs down from his tractor.

He says he has had enough of politicians ignoring the nation's farmers - a popular complaint.

"I guess you need a coffee after that journey," I suggest.

"A beer," he replies, with a weary smile. He's not expecting to do any more driving for a while.

The tractors are parked two-by-two down the carriageway, with enough space for emergency vehicles to get past.

There are trailers, too, a few vans and even a combine harvester.

Placards and slogans dangle from many of them - "our work is your food" is a typical theme.

"We are tired but determined," runs another.

Many of the tractors are decorated with the familiar rectangular signs that announce the name of a town, but which are hung upside down - an ever-more familiar sign of anger from farmers across the nation.

On the motorway, a marquee has been erected, offering chairs and warmth.

A huge pot of boiling water is simmering over a stove while potatoes and onions are prepared.

This doesn't feel like the sort of protest that will simply fold after a few hours.

Farmers, after all, are well accustomed to a bit of hardship.

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It is good natured. The police officers who accompanied the tractors are given a coffee and stop for a chat before leaving.

Lorries driving over a nearby bridge hoot their horns in support.

A man stands in a tractor shovel and is lifted to the height of a motorway bridge, so he can spray paint the name of his union.

For the moment, this is a show of strength designed to intimidate and pressurise the French government.

The assumption is that, if the farmers' demands aren't met, then they will go further towards Paris, tightening their grip on the capital.

In truth, some we spoke to seemed reluctant to commit to that, aware that the police's good humour will eventually run out if Paris is truly threatened.

That, though, is a decision for tomorrow, and the tomorrow after that.

These farmers have spent years bemoaning the fact their complaints have been ignored but now things have changed.

An army of tractors, encircling one of the most famous cities in the world, is a hard thing to ignore.