There are three different Dubais, all swirling around each other. There are the expats, like Karen; there are the Emiratis, headed by Sheikh Mohammed; and then there is the foreign underclass who built the city, and are trapped here. They are hidden in plain view. You see them everywhere, in dirt-caked blue uniforms, being shouted at by their superiors, like a chain gang – but you are trained not to look. It is like a mantra: the Sheikh built the city. The Sheikh built the city. Workers? What workers?

Every evening, the hundreds of thousands of young men who build Dubai are bussed from their sites to a vast concrete wasteland an hour out of town, where they are quarantined away. Until a few years ago they were shuttled back and forth on cattle trucks, but the expats complained this was unsightly, so now they are shunted on small metal buses that function like greenhouses in the desert heat. They sweat like sponges being slowly wrung out.

Sonapur is a rubble-strewn patchwork of miles and miles of identical concrete buildings. Some 300,000 men live piled up here, in a place whose name in Hindi means "City of Gold". In the first camp I stop at – riven with the smell of sewage and sweat – the men huddle around, eager to tell someone, anyone, what is happening to them.

Sahinal Monir, a slim 24-year-old from the deltas of Bangladesh. "To get you here, they tell you Dubai is heaven. Then you get here and realise it is hell," he says. Four years ago, an employment agent arrived in Sahinal's village in Southern Bangladesh. He told the men of the village that there was a place where they could earn 40,000 takka a month (£400) just for working nine-to-five on construction projects. It was a place where they would be given great accommodation, great food, and treated well. All they had to do was pay an up-front fee of 220,000 takka (£2,300) for the work visa – a fee they'd pay off in the first six months, easy. So Sahinal sold his family land, and took out a loan from the local lender, to head to this paradise.

As soon as he arrived at Dubai airport, his passport was taken from him by his construction company. He has not seen it since. He was told brusquely that from now on he would be working 14-hour days in the desert heat – where western tourists are advised not to stay outside for even five minutes in summer, when it hits 55 degrees – for 500 dirhams a month (£90), less than a quarter of the wage he was promised. If you don't like it, the company told him, go home. "But how can I go home? You have my passport, and I have no money for the ticket," he said. "Well, then you'd better get to work," they replied.

Sahinal was in a panic. His family back home – his son, daughter, wife and parents – were waiting for money, excited that their boy had finally made it. But he was going to have to work for more than two years just to pay for the cost of getting here – and all to earn less than he did in Bangladesh.

He shows me his room. It is a tiny, poky, concrete cell with triple-decker bunk-beds, where he lives with 11 other men. All his belongings are piled onto his bunk: three shirts, a spare pair of trousers, and a cellphone. The room stinks, because the lavatories in the corner of the camp – holes in the ground – are backed up with excrement and clouds of black flies. There is no air conditioning or fans, so the heat is "unbearable. You cannot sleep. All you do is sweat and scratch all night." At the height of summer, people sleep on the floor, on the roof, anywhere where they can pray for a moment of breeze.

The water delivered to the camp in huge white containers isn't properly desalinated: it tastes of salt. "It makes us sick, but we have nothing else to drink," he says.

The work is "the worst in the world," he says. "You have to carry 50kg bricks and blocks of cement in the worst heat imaginable … This heat – it is like nothing else. You sweat so much you can't pee, not for days or weeks. It's like all the liquid comes out through your skin and you stink. You become dizzy and sick but you aren't allowed to stop, except for an hour in the afternoon. You know if you drop anything or slip, you could die. If you take time off sick, your wages are docked, and you are trapped here even longer."

He is currently working on the 67th floor of a shiny new tower, where he builds upwards, into the sky, into the heat. He doesn't know its name. In his four years here, he has never seen the Dubai of tourist-fame, except as he constructs it floor-by-floor.

Is he angry? He is quiet for a long time. "Here, nobody shows their anger. You can't. You get put in jail for a long time, then deported." Last year, some workers went on strike after they were not given their wages for four months. The Dubai police surrounded their camps with razor-wire and water-cannons and blasted them out and back to work.

The "ringleaders" were imprisoned. I try a different question: does Sohinal regret coming? All the men look down, awkwardly. "How can we think about that? We are trapped. If we start to think about regrets…" He lets the sentence trail off. Eventually, another worker breaks the silence by adding: "I miss my country, my family and my land. We can grow food in Bangladesh. Here, nothing grows. Just oil and buildings."

Since the recession hit, they say, the electricity has been cut off in dozens of the camps, and the men have not been paid for months. Their companies have disappeared with their passports and their pay. "We have been robbed of everything. Even if somehow we get back to Bangladesh, the loan sharks will demand we repay our loans immediately, and when we can't, we'll be sent to prison."

This is all supposed to be illegal. Employers are meant to pay on time, never take your passport, give you breaks in the heat – but I met nobody who said it happens. Not one. These men are conned into coming and trapped into staying, with the complicity of the Dubai authorities.

Sahinal could well die out here. A British man who used to work on construction projects told me: "There's a huge number of suicides in the camps and on the construction sites, but they're not reported. They're described as 'accidents'." Even then, their families aren't free: they simply inherit the debts. A Human Rights Watch study found there is a "cover-up of the true extent" of deaths from heat exhaustion, overwork and suicide, but the Indian consulate registered 971 deaths of their nationals in 2005 alone. After this figure was leaked, the consulates were told to stop counting.

At night, in the dusk, I sit in the camp with Sohinal and his friends as they scrape together what they have left to buy a cheap bottle of spirits. They down it in one ferocious gulp. "It helps you to feel numb", Sohinal says through a stinging throat. In the distance, the glistening Dubai skyline he built stands, oblivious.