Kripa waits in her room. A boy slips in and hands her a small canister of pepper spray, which she tucks beneath her dupatta. She waits again. Someone else is coming. She raises the canister, holds her breath and presses the trigger. As her would-be captor gasps for air, Kripa dives for the door, running between the houses. There is a car waiting just outside the village. She jumps in and the driver guns the engine, kicking up dust as they speed away.

A few hours later she is in a safe house with Prashant, the boy she loves, the boy she was barred from marrying on pain of death because he is from the lowly Meena caste while she is a more exalted Jat. And the Love Commandos are celebrating another victory against the forces of conservatism that decree that a couple's feelings have no part to play in marriage.

It is a reflection of just how tightly caste still holds India in its grip that a group such as the Love Commandos should need to exist at all. But exist they do, a volunteer force dedicated to rescuing young lovers from families who would rather kill them than suffer the social stigma of an unsuitable match, and from the khap panchayats, the notorious village caste councils that rule on who can and cannot marry and regularly pass sentence of death on those who refuse to accept their diktats on caste or gotra (another subdivision based on lineage). The Love Commandos' phones ring night and day. What started as a group of like-minded friends protecting couples trying to celebrate Valentine's Day is rapidly becoming a national movement, with 2,000 volunteers across the country and more coming forward every day.

Founder Sanjoy Sachdev leans against the blue-painted wall of the back room of their headquarters, a small house tucked away among a warren of backstreets in the heart of old Delhi, retelling Kripa's story. His legs are stretched out in front of him on the bed that fills half the room. The 51-year-old is smoking a cigarette and talking on a mobile phone. "Welcome to the Love Commando helpline, Channel 12. Good afternoon. Sir, kindly speak in English or Hindi. Yes, yes, we can help you," he tells the caller. Sachdev draws on the cigarette, listening to the voice on the other end of the phone. It is a young boy from Andhra Pradesh, worried that his girlfriend's parents will take her away.

"If your girlfriend accepts that she is rich and you are poor, let the people in the village be angry," Sachdev tells him. "We are here to support you. Continue your love affair and call us when there is a problem. You can always call us. " It is not always so simple: sometimes they have to act fast. "When we get a call to say someone's life is in danger our teams rush to help," says Sachdev.

The volunteers – doctors, lawyers, engineers, shopkeepers, students – are unarmed save for the pepper spray. "We wanted to shun violence with non-violent means," says Sachdev. "But we use the pepper spray when someone has to be rescued and life is in danger."

The Love Commandos claim to have helped hundreds of couples marry since they set up the hotline in July. "You have to understand that in every nook and corner of the country there are couples under threat," Sachdev says. "Our society does not accept love. There is a social stigma involved in a boy and girl in love."

It was an outbreak of so-called honour killings in northern India this year that spurred the Love Commandos into action, convincing Sachdev and his friends to transform their ad hoc operation into a 24-hour national hotline. It also prompted the supreme court to issue notices to the national government and six states to protect couples from the edicts of the panchayats.

Officially, the police recorded 19 murders in northern India as honour killings between 19 April and 30 June this year. But with many such killings dismissed as "suicides", the true figure is probably about 10 times higher. One recent study estimated that there are more than 1,000 honour killings in India every year, the vast majority in the northern states of Haryana, Punjab and Uttar Pradesh.

The obsession with caste and status is nowhere better demonstrated than in the matrimonials section of India's national papers. Brahmins are much sought after by parents seeking a suitable match for their offspring. Rajputs, too. Even those claiming to be more cosmopolitan are no less demanding. "Caste no bar," one advert declared last week, while stipulating that the successful groom must be a "Nasa space scientist, age 29-30".

Those who fail to make the grade are in mortal danger if they insist on pursuing the affair: so is the son or daughter of the family opposed to the match.

Aarti found that out the hard way. The Love Commandos saved her in the end, but not before her mother had sold her three times in an attempt to get her out of the way. Better that than suffer the shame of her daughter marrying Sanjay, the boy Aarti had fallen in love with the first time she clapped eyes on him on the rooftop of the neighbouring house in the city of Agra, home of the Taj Mahal, that most familiar monument to love.

She gazes at him now as she talks, her fingers playing with the fabric of her pink and orange dupatta. The 19-year-old has a slash of vermilion running along her central parting, the sign of a married Hindu woman.

"The first time I saw him I fell for him. It was my heart's feeling. Even if I attempt to forget him, I never can," she says. He is a day labourer, aged 21, making cardboard boxes for a handful of rupees. He sits beside her in his faded jeans and cheap printed T-shirt on the bed of the safe house. "I had a clear view into her house," Sanjay says. "We used to talk to each other across the rooftops. It was love at first sight. But it was when we fell in love that the problems started."

Never mind that both their mothers worked as domestics, both their fathers were dead and they lived next door to each other: Aarti's caste, Thakur, the dominant one in the street, easily outranked Sanjay's Kashyap caste.

Three years ago, Aarti's mother, Laxmi Devi, overheard her talking to Sanjay. "It was just sweet nothings," she says. "I love you, that sort of thing."

Laxmi Devi could not contain her fury. Her daughter was consorting with a lower-caste boy and bringing shame on them all. She resolved to do whatever it took to get Aarti out of the picture.

"My cousin was beating me and my mother was beating me and the whole neighbourhood was beating me because my mother made such a hue and cry about it," says Aarti. "They just didn't want me to marry him. They said they would kill me, they would never let us get married."

She dabs at her tears. The first time her mother sold her was in 2007, for 10,000 rupees (£140). "She took me to a temple. I thought she was going to let me marry Sanjay. But a woman and a man came there. My mother was haggling with them," she says. "She wanted to sell me as a slave for extramarital relations," she says.

Aarti protested so much that the couple called her mother to take her home, and the beatings started again. "Sometimes it was a stick, sometimes with a bat, sometimes my head was banged into walls or I was slapped and punched. They used to lock me up in my room."

Last year Laxmi Devi sold her again. This time a man came to the house. Aarti was summoned. Her mother said she had decided to drop her opposition to the match. Aarti was to go with this man to find Sanjay. When the man tried to rape her, she ran to a nearby village and the women there sent her home. Again she was beaten. But her mother appeared to be tiring of the fight.

On 8 May this year, she took Aarti and Sanjay to Ghaziabad, near Delhi, presented them at a temple and told them they were now man and wife. For four days, the couple were together. Then Laxmi Devi changed her mind. Aarti was summoned home again, ostensibly to be presented to her relatives. Instead she was sold for the third and final time.

This time she found herself in Sobi Pura village near Firozabad in Uttar Pradesh. Some people had come to the house and given money to her mother. She says she was threatened with a gun if she tried to escape, but she seized her moment and ran. When she reached the road, she called Sanjay. He had heard about the Love Commandos a few days earlier on a late-night television bulletin. Now he dialled the number. The voice on the other end told him to get to Delhi immediately. When they arrived, they were taken to the safe house. The group's lawyers advised that, having already accepted each other as husband and wife in a Hindu temple, they were legally married.

"They will stay with us until we have a guarantee that they will not be attacked or assaulted," says Sachdev.

But money is tight. The Love Commandos rely on individual donations of 100 rupees (£1.40) a year from their volunteers to keep going. "We need places where the khap panchayats can't come and kill us," says Sachdev. "We are appealing to everyone who appreciates love to help us. We are branded people. We have had death threats and our effigies burned."

He flicks through the files. Here is a marriage certificate for Sandeep and Kavita from Haryana. The panchayat had threatened them and volunteers went to the village to rescue her. They got Kavita into a car and then drove the couple to a temple where the marriage ceremony was carried out immediately. The families were informed by telegram.

And here is a picture of Sunil and Geeta from Rajasthan, at their wedding ceremony in Delhi's Arya Samaj Mandir, a temple specialising in love marriages. Sonu Rangi, the group's operational commander, is in the centre of the picture, helping with the ceremony.

"It is disgusting that the government is allowing these panchayats to take place," Sachdev says. "But the sorry state is that the political leaders, for the sake of votes, have never dared, and the officers of the police have never respected the law."

The pressure for action has mounted with the number of cases of honour killing this year: a young couple, Asha and Yogesh, were tied up and tortured to death in Delhi in June because he was from a lower caste; a newlywed man, Rajesh Negi, was set alight for marrying against the wishes of his family, again in Delhi, in August; a couple, Inderpal and Maya, were killed and left in a field in Tamil Nadu two weeks ago. The list goes on. Stung by the public backlash, the Indian government has started making noises about tackling the problem, with the home affairs minister, P Chidambaram, announcing that he is actively considering a new law targeted at honour killing.

Even Bollywood is paying attention. A new movie, Aakrosh, focuses on the murder of Nitish Katara, 24, who was killed in 2002 after falling in love with Bharti Yadav, daughter of an Uttar Pradesh politician, DP Yadav. Katara was hit on the head with a hammer before being doused in petrol and set alight. "The affair was damaging my family's reputation," Yadav's son, Vikas, said in his confession to the crime.

It is a mark of just how conservative many Indians still are in their attitudes towards caste that the film's director, Priyadarshan, felt the need to stress that it was not intended as a criticism of honour killings. "We have left a question mark at the end for the audience to decide whether the practice is right or wrong," he says.

Kripa and Prashant would doubtless think the latter. They are in hiding in a safe house, 800km from Delhi. The death sentence passed on them by their khap panchayat remains in place.

Aarti and Sanjay may be married, but they are still not free. They, too, are under sentence of death. Those who defend the practice of arranged marriage often argue that other cultures fall in love before marriage and out of love afterwards, while in India they marry first and fall in love later. All Aarti and Sanjay want is a chance to find out for themselves which way is right for them.

"We love each other and we want to be together. What is so wrong with that? Why should anyone have the right to keep us apart?" asks Aarti, anger flashing in her eyes. "None should have such a mother," she says.