As clashes between Eritreans and Sudanese trying to enter the UK raise tensions in the French port, those who survived the brutal trek from Africa tell their stories

It requires expert timing, courage and a huge amount of luck. As lorries start to slow on the Rue des Garennes before a sharp right towards the port, those crouching in the shadows nearby must act quickly; several seconds is all they have to scramble across the road, prise open the vehicle's rear doors and squeeze inside.

The Rue des Garennes intersection is one among dozens of identifiable points where, between midnight and dawn, migrants – motivated by dreams of a new life in Britain – play cat-and-mouse with the French authorities as they attempt to infiltrate the port of Calais.

The French town's migrant population is growing steadily. Latest estimates reveal that their numbers have doubled to around 1,200 since the spring. Their makeshift tent and tarpaulin camps, known as "jungles", are increasingly conspicuous, and antagonising the port town's politicians. Calais has had enough.

Deputy mayor Philippe Mignonet has demanded that the border be shifted 30km to Dover and the migrants ferried north with it. Twelve years after the closure of Calais's notorious Sangatte asylum centre, the debate over what to do with sans papiers – those without documents – has again turned toxic. More broadly, the spotlight has turned to the EU's open borders agreement, which the UK refuses to sign, and which means the migrants can get to the jungles of Calais but no further. Not surprisingly, Home Office sources say they are content with the status quo.

Addressing Eritrean migrants last Thursday, Cécile Bossy of Médecins du Monde said that the intractability of the issue meant France and England could only agree on the fact that neither wanted the migrants, a situation that means most endure squalid conditions with no power or running water. "It's a problem nobody wants to face. Most are asylum seekers who want protection in Europe," she said.

The recent swelling of the cohort of migrants, including an influx of women and children, is largely blamed on Eritreans fleeing the rapacious rule of dictator Isaias Afewerki. The expanding Eritrean "jungle" has antagonised other migrant groups competing for the best places to infiltrate the well-defended port. Among the most prized targets is a large lorry park at ZI des Dunes, east of Calais, where drivers rest overnight before heading to England and which became the source of fierce rioting last week when an emboldened gang of Sudanese denied Eritreans entry.

The burgeoning migrant population and associated tension has lured hawkish elements of the British media to Calais, and their anti-immigrant coverage has dismayed many in the jungles. Robel, 24, from the Eritrean capital Asmara, said: "They tell lies, misinformation. They have their own agenda."

Another response to the unrest has been an escalation of port security. More French riot police have been dispatched to Calais. Britain has upgraded thermo-detection cameras and vehicle-scanning equipment at the ferry terminal. Bushes that used to grow beside the Rue des Garennes junction have been chopped back, eradicating cover and costing migrants valuable moments as they attempt to get on board the vehicles.

Inside the sprawling Eritrean jungle, the consensus among its occupants is that hardly anyone appears to be getting through to England. Hussain, 18, said: "It used to be 30 a week; now it is at most three." A sense of desperation is tangible. Under the glare of the midday sun, figures can be seen hanging off the port's 10ft-high perimeter fences. Groups are spotted attempting to lever open the cargo doors of stationary trucks in broad daylight. Some sneak inside, only to be found hours later.

"I was lying there thinking: 'This is it, I've made it', but the [sniffer] dogs got me," said Abit, 28, from Sudan, He had a scratch on his cheek from fighting with Eritreans. Others have given up on the lorries. Three kilometres west of Rue des Garennes, migrants attempt to circumvent security by swimming 200 metres to waiting ferries.

Mohammed, 31, from Syria, claimed he had 10 friends who had used this fraught route to reach England, though none were from Africa. "Black people do not like to swim," he said.

Back on land, another threat faces migrants: the French police. Accounts of brutality from those in the jungles are numerous. Pointing to sand dunes close to the hoverport, teenager Frezgi, from Keren, Eritrea, said: "Last night we were running from the police and they hit one of our friends unconscious. Another was hospitalised."

Bossy said: "They are using violence very often, migrants are having their arms and legs broken by police, and gas is used. Quite often the migrants say the violence is worse than it was in their own country."

Antipathy is also evident among some of the Calais public. Some shopkeepers, the migrants say, refuse to serve them. Healthcare is an issue. Despite the sizeable migrant community, only one doctor is responsible for them, as well as any French people who do not have health insurance. "People have died because they haven't had treatment in time," said Bossy.

Many have died before even reaching Calais. The route from Eritrea to the edge of England entails traversing massive tracts of the Sahara, following winding tracks through Sudan and Libya to the Mediterranean. Survivors describe days without food, of being forced to eke out a bottle of water over 48 hours. If the water runs out or their truck breaks down, death follows.

Simon, 24, from Asmara – a veteran of more than 50 failed attempts to board a lorry over the past four months – said: "Everyone died on the truck behind ours, all 21 died. They ran out of water." Others describe overloaded jeeps rolling down dunes, their occupants lost for ever.

"Two died in my truck. One woman next to me ran out of water," said Sammy, also from Eritrea's capital. His friend described how male passengers begged female travellers to hand over their urine while refusing to share theirs.

No one knows how many Eritreans have died en route. At least eight are known to have died in Calais this year. The latest might be 17-year-old Ahmed Abdullah, again from Asmara, who rang friends a fortnight ago saying he had climbed beneath a UK-bound bus, 20km from Calais, and was heading to the port. That was the last anyone heard from Abdullah.

"We don't know if he's in England or if he's dead," said Simon.

No one has told his mother that her teenage son has disappeared. One rule in the jungle dictates inhabitants must always tell their family they are safe. The going rate for passage through the Sahara is £1,700 and most families have sacrificed their savings to pay for their children's bid for a better life.

Remarkably, though, the prevailing mood among migrants is an indefatigable optimism. Everybody asked believes they will make it to England.

Tonight they will go again, weighing up whether to swim, chance their luck on the Rue des Garennes or seek weak points along the Avenue du Commandant Cousteau. Fathi, 19, who estimates he has made 100 attempts to try to board a lorry, grins: "Travel safely, I will see you in London."