It was the 13-year-old maid's desperate cries for help that finally alerted neighbours to her plight. She was standing, sobbing, on the balcony of the upmarket Delhi apartment. Her employers had locked her in, she said, and gone on holiday. Finally rescued by a firefighter, she told a tale that prompted a widespread display of national revulsion.

Her employers – middle-class doctors Sanjay and Sumita Verma – had "bought" her from an agency, which had in turn bought her from her uncle. She was hungry, she said, because they barely fed her. She received no pay and was regularly beaten. Their latest act of cruelty had been to lock her in and go on holiday to Thailand.

The couple claim that they thought the girl was 18 and deny mistreating her, but they were roundly vilified and have been refused bail. In court the couple were accused of "subjecting the victim to a treatment which can be best described as torture".

Perhaps the most puzzling aspect of the story is why it has caused such fury in a country where, after all, the sight of a youthful servant rarely raises a flicker of curiosity. Delhi's thriving middle class would crumble without its army of domestic servants, whose presence enables couples to go out to work and continue to boost an economy projected to be the largest in the world by 2050.

The most liberal members of that society think nothing of employing a maid, a driver, a sweeper, a cook, a gardener and a couple of house boys who sleep on the roof, or in tiny shared rooms.

The International Labour Organisation estimates that there are at least four million domestic servants in India, including about 100,000 children working in and around Delhi. While it has been illegal to employ anyone under the age of 14 since 2006, that has done little to hinder the placement agencies which routinely hire out trafficked children.

A good maid might earn 3,500 rupees (£43) a month, if she is very lucky, or about half the legal minimum wage for an unskilled worker in Delhi. The less fortunate are bought from brokers and kept as unpaid skivvies – simply fed and given somewhere to sleep.

A company called Domestic Help in India is one of thousands of agencies supplying staff. Based in Gurgaon, near Delhi, the company charges employers 16,000 rupees to arrange the hire of a maid for 11 months. Its website is packed with adverts for staff, who can be selected on the basis of age (15 and upwards), religion and gender. Gurpreet, a maid/cook, has two years' experience and costs 3,000 rupees a month. Harjett, who has one year's experience, is available to anyone in Delhi for just 2,000 rupees a month. Those less comfortable with the way the system operates often try to assuage their feelings of guilt by hiring staff at above the going rate.

However, writing on an expatriate website that offers advice to foreigners moving to India, Shawn Runacres, managing director of the Gurgaon-based Domesteq staff placement agency, says there should be no need to feel awkward if staff are treated well. "Throw out the guilt – remember you are providing much-needed employment at fair rates and excellent working conditions," she says. "The very thought of no longer having to make beds, cook, dust, wash dishes and do laundry sounds like heaven and, for those with children, if you add to all these things the possibility of affordable, on-tap childcare, it becomes irresistible." Speaking on Friday, she said she was convinced that the market for domestic staff would continue to grow as India's economy expanded, not least because of the challenges posed by living in India. "There are many more challenges to your daily life," she said. She doubts that it would be possible to live without staff. "You would spend your entire time just trying to keep yourself fed and your home in some semblance of shape. You can't just get water from the tap; you have to clean your water. You can't just eat fruit off the tree or out of the market. Is it a luxury? No, not in India. It is absolutely a staple of life." Runacres's agency – which does not employ children and promises fair wages and dignity of labour – pays well above the average. Others are less scrupulous.

Bhuwan Ribhu, national secretary of the Bachpan Bachao Andolan (Save the Childhood Movement), said child labour was now common in the cities, particularly involving girls aged 12 to 18, while boys aged 10 and upwards are more common in the countryside. "India cannot and must not grow at the cost of millions of childhoods," he said.

Many children are trafficked from poor states such as Bihar and West Bengal through the thousands of illegal agencies operating in the cities. Last year the movement raided a placement agency to rescue six girls and uncovered evidence of 400 girls who had been trafficked.

Ribhu says people cannot resist a cheap deal. "Well educated or not, people try to maximise their profits by employing kids. They do not pay proper wages. The probability of children leaving employment whenever they want is very low, and they may be exploited, beaten and made to work long hours," he said.

While many people in India may have been appalled by the Delhi case, he said, there were thousands of others who continued to employ children. "Children work because they are the cheapest form of labour, and in these situations they are victims of slavery. They are abused, not only economically but physically and sexually, as the exploiters also have little fear of law enforcement," he said.

Patricia Lone of Unicef says domestic labour is one of the most dangerous forms of child labour because of the potential for abuse, particularly for girls. "It is a huge problem in most countries in south Asia because of the levels of poverty."

Sometimes it is parents unable to support their children who pack them off to work; other times it is the children themselves who seek to pay their own way, she says. "But it is related to poverty, which forces parents and children to put themselves at risk."

The outcry over the Delhi maid was encouraging, said Ribhu, in that it opened people's eyes to the reality of what is going on. But he is not getting too excited about the arrests. They were, he said, an anomaly in a country where many people simply do not understand that using children as servants is wrong."Recently, I was in a mall where I saw a couple with a 10- or 11-year-old girl taking care of their baby while they were eating. When I confronted them, the lady replied that: 'She is in such a good condition here – she would starve to death in her village. Who will go feed her there? And she has even been taught English'," he said. "When I asked her if she realised that she was committing a crime, she replied that the girl was being kept just like her own daughter and she is 'even brought to the mall … can anyone in her village even dream of such a luxury, of going to the mall?' "I explained as nicely as possible to her husband that if I were to call the police to their house, they would be arrested, and if the girl was 'like their daughter', why was she not eating with them at the same table? And he had no answer."