According to the United Nations, the planet's population is currently split almost right down the middle: 3.2 billion in the city, 3.2 billion in the countryside. But by the start of 2007, the balance will have tipped decisively away from the fields and towards the skyscrapers.

No one knows for sure precisely where and when urban life started. But we can make a good guess about where the urbanising trend will reach its zenith. Simply count which skylines have the most cranes, track where the bulk of the world's concrete is being poured or follow one of the biggest, fastest movements of humanity in history. All lead east, to China.

Every year, 8.5 million Chinese peasants move into cities. Most of their destinations are mere specks on western maps, if they appear at all. But their populations put them on a par with some of the world's megalopolises. Britain has five urban centres of more than a million people; China has ninety. A few - Beijing, Shanghai, Hong Kong and Nanjing - are well known around the world. The names of many others - Suqian, Suining, Xiantao, Xinghua, Liuan - are unfamiliar even to many Chinese. Nowhere is the staggering urbanisation of the world more evident than in Chongqing. Never heard of it? This is where the pace and scale of urbanisation is probably faster and bigger than anywhere in the world today. This is the Coketown of the early 21st century.

Set in the middle reaches of the Yangtze, this former trading centre and treaty port has long been the economic hub of western China. But after its government was given municipal control of surrounding territory the size of many countries, it has grown and grown, becoming what is now the world's biggest municipality with 31 million residents (more people than Iraq, Peru or Malaysia). The population in its metropolitan areas will double from 10 million to 20 million in the next 13 years.

When the planet's rural-urban balance tips, it is as likely to happen here as anywhere. To get a snapshot, I spent a day with a Channel 4 film crew in this megalopolis - just the sort of day, in fact, when humanity might pass the halfway point on its millennia-long journey out of the countryside.

5.30am - the bangbang man

In the hour before dawn, the poor district of Qiansimen has a distinctly Dickensian feel. With the rain lashing down, puddles fill the dark, narrow alleys, flanked on either side by tall, ramshackle tenements. An old man's wrinkled face glows orange as he warms himself over a brazier.

Nestling between the port and the commercial centre, this area is the home of Chongqing's most distinctive and traditional population - the bangbang army, a 100,000-strong crew of porters who bear the city's weights on their shoulders. Arriving from the countryside with no skills and minimal education, they pick up the cheapest of tools - a bamboo pole (or bang bang) and some rope - and hang around the docks, the markets and the bus stations waiting for goods to carry up the steep slopes of this mountain port.

Yu Lebo has just woken up in the cramped three-room apartment that he and his wife share with three other couples, all of whom are porters or cleaners or odd-job men. There are two double beds in one room, separated by a thin sheet, a third in a tiny room next door and another in the kitchen. There is no time for breakfast before he heads out into the rain and the dark. "We want to move out and get a place of our own, but we don't have the money yet," he says once we are outside. He explains why he came to Chongqing four years ago. "I used to be a farmer, but I could not afford to raise my two children. So we left them behind with relatives. I see them two or three times a year."

On an average day, Yu earns about 20 yuan (£1.50) for 12 hours work. Most of this, and the money his wife earns as a cleaner, goes on rent and food, but as long as they stay healthy they can save enough to send money home to buy clothes and books for their children. It is vital. Education and health care - free in the days of Mao Zedong - are now the biggest burden on peasants.

The first job of the day is in the Chaotianmen market, where Yu must carry several huge bundles of goods. Each is probably heavier than Yu, who weighs just over 50kg. The stallholder pays him 2 yuan (15p). "Not bad," Yu says. "Sometimes they are heavier. Sometimes we get paid less."

It looks exhausting. Does Yu ever regret coming to the city? "No, my life is a little better than it was when I first got here. Then, I only earned 10 yuan a day. This city is changing so fast. It is getting richer. But our lives are not keeping up. Cities are good for the rich. If you have money you can do anything. If you don't want to carry something, you just hire a bangbang man."

7.30 am - the city official

It is just after dawn, but the sun remains hidden behind a thick haze. The giant movement of humanity that is Chongqing is about to get into full swing, working, building, consuming, discarding, developing. If today is typical, builders will lay 137,000 square metres of new floor space for residential blocks, shopping centres and factories. The economy will grow by 99 million yuan (£7m). There will be 568 deaths, 813 births and the arrival of 1,370 people from the countryside - each year, the city limits are pushed further outwards as the urban population grows by half a million, the equivalent of all the people in Luxembourg being added to the municipal register.

Our next stop is at one of the municipal offices, where Zou Xiaoping, deputy director of the economic relations commission, explains that her city is at the centre of China's drive to address the huge inequalities between the rich eastern coastline and the poor western interior.

The scale of the "Go West" policy - with 1.6 trillion yuan (£114bn) spent since 1999, mainly on roads, bridges, dams and pipelines - is sometimes compared with the Marshall Plan that helped rebuild postwar Europe. Much of the money has flowed up the Yangtze to drive the growth of Chongqing, at the heart of the plan to revitalise the west. It has also paid for the Three Gorges dam, the world's biggest hydroelectric project, which has provided the city with power and people. City residents in Chongqing have seen their incomes rise 66% in the past five years to 10,240 yuan (£731 per year), almost three times that of their country cousins on 3,800 yuan (£271).

"Now is the peak time of the development of western China. Chongqing is in the middle of it. That is why we are growing so fast," says Zou. "We must maintain momentum. This is a crucially important time for our city."

10am - the industrialist

I leave Zou's office flabbergasted. Even at the height of Britain's urbanisation in the 19th century, there was nothing to compare with the scale and speed of change taking place here. How can space and jobs be found for so many new arrivals?

Now accompanied by a government guide, we drive to the city limits and the newly built Lifan Sedan factory in the Chongqing Economic Zone, where newly employed workers are putting together newly designed cars.

"This was farmland a couple of years ago," says proud boss Yin Mingshan. "It is my 14th factory, 14 years after I started business."

A dapper, twinkly-eyed 68-year-old, Yin is one of the nation's great industrial pioneers, the 21st-century Chinese equivalent to Titus Salt, Josiah Wedgwood or the Cadbury brothers. Imprisoned for much of the Mao era for his views on free speech and capitalism, he set up a motorcycle repair company in 1992 with nine staff. His Lifan company now employs 9,000 workers and has a turnover of 7.3bn yuan (£521m).

"China has become a wonderland for entrepreneurs," says Yin. "There are many people who are doing what I have done."

It is not as easy to build a business in Chongqing as in coastal Shanghai or Shenzhen, which benefit from access to overseas markets. But those rich eastern cities are now investing inland and providing a market for the cheaper goods made in second-tier cities. Chongqing is famous for motorbikes; Yin is now also trying to make it famous for cars, by buying a BMW-Chrysler factory in Brazil, breaking it down, shipping it up the Yangtze and then rebuilding it in Chongqing. He has also set up plants in Vietnam, Thailand and Bulgaria and plans to open a research centre in Britain, where his daughter studies at Oxford.

His creed is one of benevolent self-interest. "China is too poor. We need high-speed growth. The rich need to increase the income of the poor," he says. "If we improve the living standards of peasants, then they can buy our motorcycles and cars." Within five years, he aims to more than double his workforce to 20,000. Next to the factory, bulldozers are already churning up fields for another one.

12pm - the builder

Even by the standards of the giant construction site that is modern-day China, Chongqing's building frenzy is impressive. More transport links have been built here in the past four years than in the previous hundred. More new floor space is being completed than in Shanghai. As well as eight new railways, eight highways and eight bridges, the port is in the midst of a £1.15bn redevelopment and the airport's capacity is planned to quintuple by 2010.

Driving back from the factory, I count more than 30 cranes in less than five minutes. Just outside the Jiangbei toll booth, farmers toil under heavy loads in vegetable fields and women wash their clothes in a stream. Behind them, 30-storey towers are silhouetted against the grey mist. Where the two worlds meet is a corridor of rubble where land is being cleared for further expansion.

We make an impromptu visit to the building site, where Chen Li, a brash window-fitter, reckons he has worked on 70 to 80 tower blocks in the nine years since arriving in the city at the age of 16. "The buildings are getting taller and better," he says. Yet he lives in a hut, his breakfast is a glass of soya milk and a steamed bun, and on an average day he works 11 hours for about 50 yuan (£3.60). "I'm a city resident now. But life is still difficult."

2pm - Spiderman

As people move off the land and into the sky, they produce less and consume more. In theory, they become socialised and civilised. In practice, they spend more time shopping and eating junk food. A nearby shopping centre, home to Kentucky Fried Chicken, could almost belong to any city on earth: pedestrianised streets, boutiques and fast-food outlets, a giant screen blaring out pop jingle ads, a monorail train running overhead. There are even police girls on roller skates, the latest must-have security accessory.

Li Zhiguan was once a farmer, then a factory worker; now he earns more as one of the many high-wire artists who clean skyscraper windows, earning him the nickname of Spiderman. We meet him at the top of a 24-storey telecom office just before he abseils down the glass on a rope attached to him by a single clip. "It is 100% safe. You can go too if you wish," says his boss, He Qing, with a strong German accent picked up during an MBA in Mannheim.

With so many towers going up, Li is never going to be short of work. And he has a bird's eye view of the transforming cityscape. "In six months, there have been huge changes. You can notice it from one week to the next."

3pm - the psychologist

China's growing gap between winners and losers has created an intensely competitive, restless society where stress and conflict are the norm. How do people cope? Kuang Li is a psychologist at a hospital affiliated to Chongqing University of Medical Science, where new facilities are rising on a huge construction site. She has no couch; instead, this is the most formal interview of the day in huge leather chairs in a special reception room, flanked by hospital and government officials.

Kuang is upbeat. "People have to make a big adjustment because the pace of life, work and study are all accelerating. It puts extra stress on people, but so far our research suggests they can adjust." But it is not easy. She says cases of depression, anxiety, insomnia and mood swings have doubled in the past 20 years. Between 10% and 25% of Chongqing's people suffer mental problems. Suicide appears to be too sensitive a subject to discuss; the otherwise helpful authorities decline to give statistics. But the city has launched a new campaign to prevent suicide among university students, including counselling services, a telephone hotline and free books on ways to avoid depression. Kuang says she has spent the past year researching student suicide, but she too is reluctant to give figures.

Her mental health department was established only in 1998; before that, mental problems were largely either ignored or associated with western decadence. Now, Kuang says, there is a recognition of the strains imposed by city life. "There is a conflict between rising expectations and people's sense of achievement." At the same time, she says, psychological disorders are "a sign of improved quality of life. People did not have time to worry about themselves so much 10 years ago."

5pm - the waste engineer

China's development is one of humanity's worst environmental disasters. Cheap coal and a doubling of car ownership every five years has made the country the second-biggest emitter of greenhouse gases. According to the World Bank, 16 of the planet's 20 dirtiest cities are in China, and Chongqing is one of the worst. Every year, the choking atmosphere is responsible for thousands of premature deaths and tens of thousands of cases of chronic bronchitis. Last year, the air quality failed to reach level 2, the government health standard, one day in every four. Today's haze is so thick that I still haven't seen the sun.

Chongqing is trying to clean up, but this is a low priority compared to economic growth. And it is hard to find a place for the ever-expanding waste. We head into the hills to see the biggest of the mega-city's rubbish mega-pits: the Changshengqiao landfill site. It is an awesome sight; a giant reservoir of garbage, more than 30 metres deep and stretching over 350,000 square metres.

The waste engineer, Wang Yukun, tells me the city produces 3,500 tonnes of junk every day. None of it is recycled. Some is burned. Here, it is layered like lasagne: six metres of rubbish, half a metre of earth, a chemical treatment and then a huge black sheet of high density polyethylene lining. The site opened in 2003 and it already contains more than a million tonnes of rubbish.

"It was designed to serve the city for 20 years, but it has filled faster than we expected. I guess it will be completely full in 15 years," Wang says. "Once it is finished we will build a golf course on top."

6pm - the cop

In many Chinese cities, the public security bureau is more likely to detain journalists than to take them for a drive. But in Chongqing, the city goes so far as to dispatch an English speaking officer, Lai Hansong, as our guide. Lai insists he is a regular beat cop, who has been patrolling the Yuzhang district for the past six years. "It is a low-crime area," he says. "We mostly deal with thefts or fights." In an average week, he says, he deals with fewer than five incidents.

It is not what I expected, having heard lurid stories of drugs, prostitution and organised crime. The city has also been the focus of violent industrial protest. Last November, 20 strikers required hospital treatment after police broke up a 10,000-strong protest over lay-offs from the Tegang state-owned steel factory. Less than a year earlier, police cars were torched and overturned in a riot by thousands in the satellite city of Wanzhou.

The picture Lai paints is very different: "There are no criminal gangs in China. Our country has few riots." But someone must be worried about something. The police force, Lai says, is increasing every year and officers must travel three to a car.

8pm - the intellectuals

This is a city that dazzles when night falls. Multi-coloured illuminations light up everything from the housing blocks that rise up on the hillside to the giant city centre replica of the Empire State Building. Motorway crash barriers glow pink, green and purple. The swirling surface of the Yangtze reflects the glow.

In a riverside restaurant I am meeting some of the city's alternative thinkers. What do they make of the place? The group laughs at the notion that there are no gangsters and some shake their heads at claims that the haze is just bad weather. Overall, they feel living standards have improved. Cultural development might be slower than material development, "but this is a city of the future," says Li Gong, a poet and cartoonist.

"Compared with 10 years ago, the air quality is better. But compare it with other cities in China or other countries and we are still far behind," says Wu Dengming, an environmental activist who founded the Green Volunteer League, which has highlighted many of the problems of the Three Gorges dam.

Zeng Lei, a documentary maker who spent seven years recording the lives of Chongqing's poorest residents, relates unhappy anecdotes of urban life - the bangbang man who burst into tears when he returned to his home village for the first time in three years; the housewife who felt so neglected by her family that she hired a team of bangbang men to carry banners through the city celebrating her birthday.

Song Wei, a publisher, notes that the evident problems - pollution, loss of heritage, inequality and crime - are not confined to Chongqing. "We could be talking about almost any city in China."

10.30pm - the new rich

Or for that matter, almost any city in the world. Chongqing is not just urbanising, it is globalising. Little more than a generation ago, this was a city where Red Guards in Mao tunics chanted anti-imperialist slogans. Today, young people with money dress much like their counterparts in Birmingham, Chicago or Nagoya. If anything, their values are even more materialistic.

I am sitting in Falling, which Spiderman's boss He Qing recommended to me as the hottest nightspot in Chongqing. It is Wednesday night, but the dancefloor is packed with beautiful people moving to techno music. Our table has an 800 yuan (£57) minimum charge, which covers a bottle of vodka, a few imported beers and a plate of elegantly carved fruit.

He joins us, along with some of Chongqing's new rich, including the founder of a sweet factory, a restaurant owner and a bank employee. Almost without exception they are in their 20s, foreign educated and well connected - either through family or political ties - with the city's movers and shakers. "No businessman can thrive unless they have contacts in the Communist party and the underworld," I am told.

I feel uneasy spending more on a night's entertainment than bangbang man Yu earns from a month's gruelling work. I'm not the only one conscious of the gap. Qing tells me his plan for the future. "Inequality and environmental destruction are the two biggest problems facing China." He says he wants to establish a new clean-energy company that will employ more migrants to build a cleaner city, using German technology.

00.30 - the street kid

Outside at midnight, the bright lights cannot mask a seedier side of city life - the poor trawling through rubbish bins, the homeless on street corners, the touts offering drugs and sex for sale. Many of the women working as prostitutes are rural migrants. Their children are left with relatives or sent to the streets to beg, sell flowers or sing songs for money until the early hours.

At a night market, a queue of hawkers comes to my table to offer to clean my shoes, sell me cigarettes or pour me soup from a flask. A seven-year-old girl plucks at my arm and then coyly entreats me to buy a rose from her. "Where is your mother?" I ask. "Oh, she's at work," the girl replies.

A desperate-looking girl is carrying a menu of songs and a battered, badly-tuned guitar. She says she is 16 but looks more like 12. She has been in Chongqing only a few months and has already decided she does not like it. I pay 3 yuan (20p) and pick the song Pangyou (Friend). The young busker stares at some faraway point as she strums the one chord she knows and sings out of tune. It is miserably sad. Further along the street, a bangbang man wanders into the distance carrying his bamboo pole. I wonder if he is about to finish work or start it.

· Additional reporting by Huang Lisha. Jonathan Watts' film about Chongqing for Guardian Films and Channel 4 News is broadcast tonight on Channel 4 at 7pm