The spate of suicides made headlines around the world. Last May, seven young Chinese workers producing Apple iPads for consumers across the globe took their own lives, prompting an investigation into working conditions at the Foxconn factory in Shenzhen, southern China.

Nine Chinese sociologists wrote an open letter to the media calling for an end to regimented and restrictive work practices which they condemned as "a model where fundamental human dignity is sacrificed for development".

One year on, swaths of anti-suicide netting surround the huge worker dormitories in Shenzhen. But an investigation by two NGOs reveals that many workers making iPhones and iPads for eager world markets are exploited and living a dismal life.

In Shenzhen and Chengdu a joint Foxconn workforce of 500,000 is providing the labour that, in the first quarter of 2011, contributed to Apple Inc net profit of $6bn (£3.6bn). Interviews with mainly migrant employees and managers have laid bare the dark side of those profits: a Dickensian world of work that would be considered shocking in the west.

The Centre for Research on Multinational Corporations and the human rights group Students & Scholars Against Corporate Misbehaviour (Sacom) have a track record in investigating the human cost of China's economic boom. The interviews they recently conducted in Shenzhen and Chengdu, which have been passed to the Observer, are sometimes heartrending.

"Sometimes my roommates cry when they arrive in the dormitory after a long day," one 19-year-old girl told investigators. "It's difficult to adapt to this work and hard to be away from your family."

Li (not her real name) arrived a few months ago to join the rapidly growing workforce at the newest factory opened by Foxconn, which is Apple's major supplier. She was attracted, like many of her colleagues, by government adverts promising work and good pay.

Apple is publicly committed to good employment practice. Its supplier code of conduct demands that employees in its supply chain are treated with respect and dignity. But Li claims that her experience has been one of illegally long hours and draconian rules for a basic daily wage of as little as £5.20. Like her, many Foxconn workers manage to go home only once a year.

For the first few days at the factory, Li said that she and her colleagues – most seem to be aged 18-20 – were put through military drills by former soldiers: "They made us do marching and standing still and walking. It was very boring."

The dormitories where she and most others live offer little comfort. Up to 24 people can share one room and the rules are strict, even prohibiting the use of a kettle or a hairdryer. One worker who did was forced to write a confession letter: "It is my fault. I will never blow my hair inside my room. I have done something wrong. I will never do it again."

Many workers interviewed claimed that they were regularly required to work far in excess of the 36 hours of overtime per month that Chinese law – and therefore international labour law – permits. At Chengdu it was claimed that anything between 60 and 80 hours of overtime a month was normal. One worker produced a payslip showing 98 hours of extra time in a single month – nearly three times the legal maximum and in breach of Apple's own code of conduct. The rule that employees should have one day off in seven is often flouted, some claimed.

Others said that if they missed targets, they had to work through their lunch breaks to make up for it. When they do get a day off, they spend much of it catching up on sleep. During work, some employees claimed they were forbidden to speak to each other and some were forced to stand for hours without a break. Foxconn, a Fortune 500 company, does not deny it breaks the overtime laws, but claims that all overtime is voluntary.

Workers who step out of line can be publicly humiliated, it is alleged. "When a worker makes a mistake, when he talks or laughs loudly, he will be humiliated," a production worker said. "Sometimes you have to stand like a soldier in front of everybody. It is a loss of dignity and means an extra pressure for the worker."

A typical working day in Chengdu means getting up at 6.30am, catching a bus for the 30-minute ride to the factory at 7.10am and attending a compulsory – but unpaid – assembly at 8.10am, before starting work at 8.30am. Shifts, including overtime and breaks, end at 8.30pm. Night shifts follow a similar pattern; with demand for the iPad2 outstripping supply in many countries, this is a round-the-clock operation. Demand for the first iPad was so intense that workers claim they had to put in a seven-day week during peak production period.

"We only had a rest day every 13 days," claimed one. "And there was no overtime premium for weekends. Working for 12 hours a day really made me exhausted." Sacom says the company's initial response to the suicides was to bring in monks to exorcise evil spirits. The chief executive later suggested workers were committing suicide to secure large compensation payments for their families. Workers were even asked to sign a document promising not to commit suicide and pledging that if they did their families would not claim more compensation than the legal minimum.

Eventually, the company raised wages at Shenzhen, though it is currently switching much of its production to Chengdu, where it expects to eventually employ 200,000 people. There are about 400,000 workers at Shenzhen, a number expected to drop to around 300,000.

The company concedes that it has faced "some very challenging months for everyone associated with the Foxconn family and the loss of a number of colleagues to tragic suicides".

The anti-suicide nets, says Louis Woo, special assistant to the chairman, were suggested by psychologists and other suicide prevention experts. The anti-suicide pledge, Foxconn said, was the idea of the official employee labour union at a rally last year and was "dedicated to the promotion of treasuring your life". Critics point out that in China, unions are not independent bodies.

When the Observer challenged Woo over the NGOs' findings, he said the workers' criticisms were unduly harsh. Responding to the allegations of public humiliation, he said: "It is not something we endorse or encourage. However, I would not exclude that this might happen given the diverse and large population of our workforce. But we are working to change it."

Not all employees had to stand, he said, and there was no "ban" on talking in the factories. Instead, employees were "encouraged not to engage in conversations that may distract them from the attention needed to ensure accuracy and their own safety". NGO investigator Leontien Aarnoudse is unimpressed. Although Apple and Foxconn make enormous profits, she said, employees "work excessive overtime for a salary they can hardly live on while they are inhumanely treated by the management.

"The work is so monotonous and they are so young. When they start this job they have no idea what they are letting themselves in for. They don't have a social life any more. Their life is just working in a factory and that is it."

While Apple says it expects high standards from suppliers, its own audit reports suggest that many fall short. The latest figures show Foxconn's Chinese factories are not alone in working staff beyond the legal limits, with fewer than one in three supplier factories obeying the rules on working hours. The audits also show that 30% broke rules on wages and benefits, while 24% were in breach of strict rules on involuntary labour.

In Shenzhen and Chengdu, the workforce knows only too well that such conditions can all too often lead to despair and, last summer, to tragedy.In a statement, Apple said: "Apple is committed to ensuring the highest standards of social responsibility throughout our supply base. Apple requires suppliers to commit to our comprehensive supplier.

• This story was amended on 1 May 2011 to correct the name of The Centre for Research on Multinational Corporations