“Contractors came to our village and said we would earn good money if we came with them to work in a quarry. But we were never paid any wages,” she said, speaking in Delhi .

“They paid us 500 rupees (£6) at the beginning and that was all. After that we occasionally got a few rupees for food. When we tried to leave they said we had to leave behind one of our children in case we ran away.”

More than 100 other forced labourers including 45 children from central India had similar tales to tell. Children as young as seven worked 12 to 14-hour shifts breaking stones in a quarry in western Haryana, a state bordering Delhi .

The released workers complained that guards beat them and sexually harassed female workers. When contacted by telephone, Virendra Singh, a local magistrate involved in investigating the case, claimed the allegations were false and it was simply a minor pay dispute.

“This is the standard reply,” said Shri Chaman Lal, the special rapporteur for bonded labour at the National Human Rights Commission. He said that India has strict laws to punish employers but they are poorly implemented. Cases rarely go to court, he added, and those that do usually result in acquittal.

The worst offenders are found in agriculture, carpet weaving, brick kilns, quarries and construction sites. Increasingly, children are also trafficked as cheap domestic labour for middle-class homes.

According to Lal , India 's local and state authorities are not tackling the issue seriously as they cannot admit the law is being flouted on their patch. In some cases, he said, there is collusion between officials and corrupt employers.

It is difficult to measure the true extent of forced labour in India . While government figures indicate 285,000 people have officially been released from forced labour, the unofficial figure is far higher.

The International Labour Organisation (ILO) estimates that 9.5 million of the world's 12 million forced labourers are in the Asia Pacific region, the bulk of them thought to be in India . Civil rights groups say the figure is in the tens of millions.

Some 80% of forced workers are Dalits – the traditionally oppressed classes considered below Hindu castes. They tend to be illiterate, innumerate and unregistered and include a large proportion of India 's 30 million seasonal migrant workers. Fleeing poverty and drought-stricken areas, they are often targeted by unscrupulous contractors and loan sharks.

“People would be guaranteed a job somewhere and then be economically exported,” said Julian Parr, an ILO adviser on bonded labour. “Very often the loan is as little as their train fare. Once they've borrowed the train fare they have to pay interest on it, then they're locked in the debt cycle.” Parr said some people were tricked into bondage, but others were born into it, as in feudal times.

Those wishing to escape find the odds against them as they lack community and family ties which could provide a safety net. Outside help is generally the only way out.

“Employers have a very strong political lobby. They contribute to political parties even at town and village level. If you upset them you're going to have a very difficult time,” said Parr.

With no fixed address and no voting rights, the forced workers are shunned and seen of little political significance, he added.

The South Asian Coalition on Child Servitude (SACCS) has carried out about a dozen raids in the past nine months, freeing almost 600 forced labourers, most of them children.

Kailash Satyarthi, chairman of SACCS, has earned a reputation for dramatic “raid and rescue” operations in quarries, sweatshops and factories employing forced child labour.

“Raids raise awareness and promote a sense of self-confidence in the laws – not only among those who are liberated,” said Satyarthi. “When people go back to their villages they spread the message. And then people understand that freedom is possible and there are laws there to help them.”

Satyarthi informs the police a raid will take place only when it has begun so employers are not tipped off. “If you go through the bureaucracy in India it's so difficult and so long,” he said.

“It's not just a question of going to the police and asking them to help. They take bribes and have no authority to liberate bonded children unless they involve a magistrate.”

The Bonded Labour Liberation Front (BMM) carries out similar raids and has released more than 172,000 people in the past two decades.

The majority of those released are not formally registered as former bonded labourers and do not benefit from government compensation schemes. They rely on groups like SACCS and BMM for support, but many fall back into the same kind of jobs .

Swami Agnivesh, head of the BMM, believes the government must take radical steps to prevent forced labour from increasing as the Indian economy continues to boom. He estimates at least 370m Indians are in forced labour, living on less than a dollar a day.

Without schemes to enforce minimum wages and provide work and education for the poor, he said nothing would change.

“I feel we can't go on identifying bonded labourers, conducting raids, and prosecuting the employers,” said Agnivesh. “There's no end in sight in a country where half the population is reeling under conditions of modern-day slavery.”

A recent employment scheme which promises 100 days of work and a minimum wage for every rural family may pave the way forward – if it is fully implemented. It is still unclear how it will be financed and if it can deliver its promise.

“The biggest political task facing this country today is to organise and give voice to the voiceless,” said Agnivesh. “At the moment they are being virtually suppressed.”