CALAIS, France — With whoops and with whistles, they emerged from the “Jungle” just as the unseen sun dipped low on a horizon painted in shades of gray.

At first, just a few clambered up the muddy embankment leading to the highway. Then dozens. Then a crowd of hundreds, all desperate to halt the speeding traffic long enough that maybe a lucky few could stow away on a truck. From there, the dream that has drawn thousands of asylum seekers to this gritty French port town would lie within reach: a new life in Britain.

But the riot police were ready. As rocks rained down on the helmeted officers, tear-gas canisters came sailing back. Coughing and running through toxic plumes, the crowd retreated to the Jungle, that cauldron of poverty, disease, violence and frustrated ambition.

Calais, separated from Britain’s White Cliffs of Dover by a mere 21 miles, has been the reluctant host of a multinational migrant colony for well over a decade. But ever since Britain and France enacted much tougher security measures at the end of August, it has become a pressure cooker with no release valve.

Day by day, more people move in, one stream in the rivers of humanity that have flowed this year from the Middle East, Africa and South Asia, inundating Europe.

With miles of new barbed-wire fences, squadrons of heavily armed police and packs of specially trained sniffer dogs all contributing to this town’s fortresslike feel, fewer and fewer people are getting out.

A COMBUSTIBLE MIX

The result is a combustible mix: a growing population of people who don’t want to be here and who despair that they will never be able to leave. More than perhaps anywhere else on Europe’s migrant trail, the mood here is increasingly defiant, with asylum seekers becoming ever bolder in their attempts to challenge security forces head-on and break through the tightening cordon that prevents them from reaching Britain.

One of the most troubling incidents for authorities came last weekend, when 200 migrants broke into the Eurotunnel’s French terminal, clashing with police and prompting an hours long suspension of the undersea rail service linking the two countries. John Keefe, a Eurotunnel spokesman, said the “very aggressive and very organized” attempt represented “a different kind of tactic.”

“If they can’t get through on their preferred route without violence, will they then use violence to get through?” he asked. “There has never been any anger before, but this was, ‘We don’t care. We are going through.’ ”

The grim conditions in the Jungle – the name the migrants have given their sprawling encampment on the outskirts of town – help explain why they’re so determined to get out.

Set in flood-prone scrubland within sight of the English Channel, whiffs of salty sea air mix with the scents of human waste and burning trash. People who have fled war or despotic regimes in Syria, Iraq, Afghanistan, Eritrea and Sudan find themselves sleeping on cold, damp French soil, with only leaky plastic tents to shelter them from the elements. Inter-ethnic rivalries and disputes over black-market business deals lead to frequent eruptions of violence.

‘THE WORST SLUM’

“It’s the worst slum in Western Europe – maybe even in all of Europe,” said François Guennoc, a camp volunteer with the aid group L’Auberge des Migrants. “The French and British governments should be ashamed of this situation.”

And yet, it is looking ever more permanent, with makeshift shops selling everything from cellphones to bicycles; restaurants; three mosques; a church; a theater; and even several “hotels” that offer, for a price, a bit of padding to sleep on and guaranteed protection from the rain. Whereas migrants once moved in and out of Calais in a matter of days or weeks, many have found themselves stuck here for months. The population is estimated to be at least 4,000 and grows by the day.

For Nawaf, a 23-year-old camp newcomer, “every day in the Jungle is like a year.”

“All my life, I’ve slept in my own bed with pillows,” said Nawaf, a stylish Syrian who sports hipster glasses and a black leather jacket. “I’ve never dreamed of something like this.”

And yet, Nawaf, who declined to allow his last name to be used because he fears for the security of his family in Syria, said he remains determined to reach Britain – at least for now.

Like many here, Nawaf said he believes life in Britain offers better career and education prospects than anywhere else he can go in Europe. Plus, he already speaks English.

“Germany and Austria offer a very good life,” he said. “But they have a very hard language.”

So as Nawaf followed the migrant trail this fall, he bypassed those two countries and kept going until he hit the end of the road in France. From the Jungle, he can see the British coast – but getting there is another matter.

Both day and night, he walks for hours along train tracks, scampering over fences and seeking to evade the notice of police officers.

On one of his first nights in Calais, he managed to slip into the undercarriage of a truck bound for Britain. The smell of turning gears and racing asphalt mingled with the Versace Eros perfume that Nawaf wears, believing that dousing himself in the sweet-smelling spray will throw sniffer dogs off his trail.

But when the truck pulled up to a checkpoint, the police weren’t fooled.

“Come on!” barked the officer, shining a flashlight in Nawaf’s eyes and motioning for him to exit. He did – and then started looking for another truck.