"The mother couldn't bring the dead body and she doesn't want to leave it as well. She was standing … holding the baby. She didn't know what to do … At the end, because of the shell bombing and people rushing – there were thousands and thousands of people, they were rushing in and pushing everyone – she just had to leave the baby at the side of the road, she had to leave the body there and come, she had no choice. And I was thinking in my mind 'What have the people done wrong? Why are they going through this, why is the international government not speaking up for them? I'm still asking."

Four months later and Gnanakumar is sitting on a cream leather sofa in the living room of the family home in Chingford, Essex, reliving the final days of Sri Lanka's brutal civil war.

For most of those four months, the 25-year-old British graduate was imprisoned behind razor wire inside the country's grim internment camps, home to nearly 300,000 people. She was released last week, partly as a result of pressure from this newspaper, and flew back into London on Sunday.

The last time she publicly spoke about the conflict was from the hospital where she was working inside the ever-shrinking war zone in Sri Lanka's north-east. Then, the national army had surrounded the small sliver of land where the remnants of the Tamil Tiger guerrillas held out and where hundreds of thousands of civilians had taken refuge. She had been in despair: a shell had just struck the hospital and dozens were dead. "At the moment, it is like hell," she said then.

Gnanakumar was one of a small group of medics treating the wounded and providing a running commentary to the outside world from behind the lines. For months she had managed to stay alive while around her thousands died. At night, she lived in bunkers dug in the sand. During the day, she helped in the makeshift hospitals, dodging the shells and the bullets, tending the wounded and the dying, as the doctors tried to operate with butchers' knives and watered-down anaesthetic.

Now her damning account provides a powerful rebuke to the claims of the Sri Lankan president, Mahinda Rajapaksa, that the defeat of the Tamil Tigers was achieved without the spilling of a drop of civilian blood.

Born in Jaffna in the Tamil-dominated north of Sri Lanka in 1984, Gnanakumar and her family moved to Britain in 1994. Until 28 February last year, she had not been back. She had just completed a biomedical degree at Greenwich University, but her short-lived marriage was on the rocks and she decided it was time to make a clean break. She left the house, telling no one where she was going.

Arriving in the capital, Colombo, she headed for Vanni, the Tamil heartland, to stay with a relative she calls her brother (her real brother is back in the UK, along with her two sisters). There seemed little sign of danger, but by June 2008 fighting was getting worse: the Tamil Tigers, or Liberation Tigers of Tamil Eelam (LTTE), still thought they would be able to negotiate a ceasefire, as they had done in the past, but the government had other ideas. They were determined to destroy the LTTE once and for all. Gnanakumar decided to stay on to try to help those who were trapped by the advance.

Even before the arrival of the government's ground forces, there had been regular air raids by air force Kfir jets. But in early January artillery barrages began, forcing the population to move.

That was when the reality of the war hit Gnanakumar for the first time.

"It was raining and … you could see everywhere on the road the blood is running with the water and the bodies were left there because there was no-one to identify who was dead and who is alive, the bodies were just laid down on the floor and that's the first time I saw dead bodies and wounded people crying out, shouting."

Wherever they stopped, they built a bunker, digging down until they could stand up in the hole, cutting down palm branches and laying them across the top for a roof and packing sandbags on the top and around the sides.

As the frontline advanced, trapping as many as 300,000 people inside a shrinking enclave of LTTE-held land, Gnanakumar went to the makeshift government hospital, which had moved into a former primary school, and volunteered to help, dressing wounds and administering first aid.

Her laboratory training had not prepared her for anything like this, but she learned as she went along. As the fighting intensified, they were treating as many as 500 people every day in two rooms. "They had a shortage of medicine but they had to somehow save the people. The last two weeks or so there was a shortage of everything."

With replacement blood running out, she had to filter what she could from the patients through a cloth before feeding it back into their veins. When the anaesthetics ran short, they diluted them with distilled water. "I watched when there was a six-year-old boy," she said. "They had to take off the leg and also the arm, but they didn't have proper equipment, they just had a knife that the butchers use to cut the meat, and we have to use that to take off his leg and arm. He cried and cried."

As the army closed in, it got worse.

"People were running and running to get them safe away from the shell bombing, but they couldn't and it came to a point where we thought we are all going to die, there is no way we can be safe anymore here, but we just have to take it. I mean, you can't get out of the shell-bombing. I didn't think that I would be alive and I would be here now. I said OK, I'm going to die, that is the end of it.

"One day I was inside the [operating] theatre and the next room was bombed. We had a lot of the treated people left in the room for the doctors to go and monitor and they all died in that shell bomb. And they [the Sri Lankan forces] again bombed the hospital and one of the doctors died in that."

Inside the hospital, there was no respite. Gnanakumar cannot forget the day a mother was brought in, injured, clutching her baby.

"She had the baby on her lap, the baby is dead and the mother didn't know and the doctor said: 'Don't tell her, because if we tell her now she will start crying out and shouting and … we have to save the mother first.' So we said: 'OK, give the baby to us, we'll look after her you go and get the treatment from the doctor,' and only after she got the treatment we told the truth, that your baby is dead. I can easily say it, but at that moment I was in so much pain, the innocent baby, the mother didn't know the baby was dead, she thought 'my baby is sleeping'.

"There were so many incidents. Another time the mother was dead and the baby was still suckling."

The fighting was getting closer. They ate what they could find and slept, those who could, in the occasional lulls.

"You have to be ready to run, you can't relax and go to sleep, any minute you just have to be ready," she said.

Gnanakumar could not take any more. On 13 May the hospital had been hit, killing about 50 people. "The bunker right next to ours had a shell on top of it and there were six people in the same family died and three were wounded.

"I saw them … suddenly I start hearing people are crying out and I thought, it has to be somewhere really close … I came out of my tent and I saw blood everywhere and the people – I couldn't even imagine that place, there was blood and then the bodies were into pieces everywhere and my brother said: 'Just pack up and let's get away from this place.'"

In the last five days, she says, she believes about 20,000 people died. It is a very high estimate, though the UN has acknowledged the true death toll may never be known. Tamil groups such as the Global Tamil Forum say her account corroborates their own figures drawn from interviews with survivors.

Over the course of the three-decade war, it is estimated that up to 100,000 people died. But independent confirmation of the death toll in the final days has been impossible. The Sri Lankan government has barred independent journalists from the war zone to this day, and has expelled UN officials and aid workers.

Meanwhile, the survivors of the final assault have been spirited away inside sprawling camps in a militarised zone.

It was to those camps, at Menik Farm, that Gnanakumar was taken. Following that last bombing, she joined thousands fleeing towards the government lines. "We start moving and after walking about one hour or so we saw the Sri Lankan army. They were saying: 'Come, you are safe now, food will be provided for you.' There were bodies everywhere, like into pieces. We had to just walk." That was when she saw the mother agonising over what to do with her dead baby. No one had time to bury the bodies, she says. Some pushed them into bunkers and covered them with a little sand. That was the best they could do.

That night, they slept in a school, then they were taken by bus to the town of Vavuniya. She called her mother: "I said, Mum, just get me out of here, I just want to get out of this place. And the phone got cut off."

The Sri Lankan government has built a series of camps to house the estimated 300,000 people who poured out of the war zone. It claims that it needs to hold the civilians until it can weed out the former Tamil Tiger fighters; its critics, including many UN organisations and independent aid groups, question why, even if that is true, it needs to imprison children and the elderly behind barbed wire, and why it has not more quickly identified the rebels. Despite pledges to start sending the internees back to their homes "at the earliest possible opportunity", the UN says only 2,000 have so far been released.

There was no food the first day Gnanakumar arrived, and she had lost contact with the people she had been with. She slept in a tent with strangers.

Even after the privations of the war zone, conditions in the camp still came as a shock.

"Wherever you go there are big queues, whatever you want you have to queue. The toilets are terrible, I can't describe how disgusting. Flies everywhere, mosquitoes, unhygienic … People had all sorts of illnesses.

"People have lost their family members, they are separated from their families … and they are going through depression."

Accounts circulated of rapes and murders, of people disappearing. Some people committed suicide: a teacher was found hanging from a tree.

Military intelligence officers were roaming the camps, looking for former Tamil Tigers, she said. "It is an open prison, you are free to walk but you are inside a prison, you are not allowed to step out. You can't. There were guards everywhere and checkpoints."

A couple of days after she arrived, the British high commission made contact through the UNHCR. An appeal from her parents in the Guardian brought fresh hope and a flurry of activity: she was moved from the overcrowded zone two to zone one, the part of the camp the authorities show to visitors.

"I was there when the UN secretary Ban Ki-moon came in … He stayed there for about 10 minutes and just went. Why didn't he go into the camp and talk to the people and spend some time asking them what their problems were? I thought he has a responsibility and people were expecting something from him. They expected much from him and he just spent 10 minutes and that's it."

The officials told Gnanakumar she would be staying for a couple of days and would then be released. "And then the 48 hours turned into three days and then it turned into weeks and months and I thought OK, now I understand it is not going to happen." She was interrogated five times – what was she doing there? Why had she been in the hospitals?

The call to say she was going home came last week. She was taken to Colombo to meet the president's brother, Basil Rajapaksa.

"He said OK, you went through so much in the country and now you are released you can go and join your family and be happy. He wasn't sorry about it." She was then handed over to British officials.

She speaks in a matter-of-fact way, rarely betraying emotion. Her hair has been tied back tightly – she had beautiful hair before she left, she says, but lost most of it in the camps. She is not sure what she will do now, maybe something in the field of medicine.

"I'm happy and proud of myself that I was able to help the people. I still think it is unreal that I am in the UK … I never thought I would be alive and coming back, even in the camp.

"After looking at the people dying and dead bodies everywhere, it is like nothing threatens me any more, it is like I have had the hard time in my life and I think I am prepared to take up whatever happens in life now.

"I'm not that old Vany that sits down and cries for little things. I'm stronger now after going through and seeing all that problem. My mind is clear now." [courtesy: Guardian UK]