The following correction was printed in the Guardian's Corrections and clarifications column, Thursday 19 November 2009

We said in the article below about the Dignitas centre in Switzerland that the director of public prosecutions for England and Wales, Keir Starmer, is "working on reducing the likelihood of being prosecuted for assisting a suicide". The Crown Prosecution Service would like to make clear that the publication of new interim guidelines by Mr Starmer's office does not reduce the likelihood of someone being prosecuted but rather details the public interest factors, for and against prosecution, to be considered in such cases.

Ludwig Minelli is explaining the best techniques for an efficient suicide when the doorbell goes and he pauses to answer via an intercom. It is already dark outside his cluttered, dimly lit conservatory, and heavy rain is beating at the glass roof. "Would you excuse me for a moment?" he says, frowning at the interruption. "A taxi driver tells me that Greek persons are coming and they want to speak with me."

Ten minutes later he reemerges, shaking out his black anorak which is glistening with rain. "It's absurd," he says, with an embarrassed laugh. "A Greek lady and her uncle, knowing not a single word of German and no English have come to Zurich." Standing on his doorstep in the pouring rain, the Greek woman has somehow made it clear that she would like him to help her to die.

Such peculiar intrusions happen every month or so because Minelli, 76, is now famous around the world as the founder of Dignitas, the not-for-profit assisted suicide organisation that has helped 1,032 people to die since 1998. He tells anecdotes, with black humour, of other unexpected visitors who arrive, hoping to die. A few months ago, as he was driving home, he saw a German taxi parked at the side of the road, the driver asking a passer-by for directions. "I stopped because I knew there could only be one person they were looking for," he says. Inside there was a woman in her 90s who had taken a 300km taxi ride from Munich and who told him: "I am now here."

Another time there was a young man from Germany, only 20 but profoundly depressed, who rang him and said: "I am in front of your house. I want to die, immediately."

"I do not like these incidents," Minelli says. "It is not very agreeable either for me or for the people looking for help." He has sent the Greek woman away, telling her he cannot help her since she has made no appointment, but he is dismayed at the suffering that has driven her to travel from Athens to seek out his home in a suburban village outside Zurich, and mutters: "Deplorable."

There are established procedures that must be followed in order to receive Minelli's assistance in securing a swift death with a 15mg dose of a lethal drug. Merely turning up on his doorstep is not the correct way.

First, you need to become a member of Dignitas; anyone can join if they pay an annual fee of 80 Swiss francs (£47). When you are ready to die, you need to send in copies of your medical records, a letter explaining why things have become intolerable and £1,860. These files are dispatched to one of Dignitas's affiliated doctors, who considers on the basis of the medical history whether or not he would be ready to write a prescription for the fatal dose. If he agrees in principle, then a "green light" is given to the member, and they can contact staff at the Dignitas headquarters, who will schedule a date and offer advice on hotels. Once they arrive in Zurich, the individual must pay £620 for two appointments with the doctor (to check their records and prescribe the drugs) and a further £1,860 to pay for two Dignitas staff members to organise and witness the death. Those who cannot afford the fees may pay less.

Since Swiss law allows assisted suicide, but not euthanasia (the difference being that the person who wants to die must actively take the dose himself), the act of voluntarily drinking the drug, mixed with 60ml of water, and the subsequent death is videoed by the Dignitas companions, who stay behind to deal with the police and the undertakers in the hours that follow. For those unable to lift the glass to their lips, there is a machine that will administer it, once they press a button.

In the months leading up to the death, Minelli and his colleagues repeatedly question whether the individual really wants to die, and set out alternatives to suicide. "It is quite simple. As long as we are able to help them in the direction of life, we help them in the direction of life," he says. When this fails, "We are ready to help them in the other direction."

The vast majority of people who visit Dignitas are the terminally ill or those with an incurable, progressive disease. "Usually, if the person has terminal cancer, motor neuron disease or multiple sclerosis and they are telling us 'I don't like to live some weeks or months until the terrible end', then it is quite clear and we have no difficulty in saying yes," Minelli says.

Then there are those people who are just tired of life. With life expectancy growing and medical sophistication improving, people are increasingly worried about whether they will be "condemned to linger on", Minelli says, "forced to end their lives in an institution. Our members say: with our pets, when they are old and in pain, we help them. Why am I not entitled to go to the vet? Why haven't I such an opportunity? We hear this often."

But it is not always as simple as he suggests. Minelli's vision goes beyond helping the infirm to shorten a painful end; his views are much more radical. He believes the right to choose to die is a fundamental human right and, in theory, he is willing to help anyone.

News that the conductor Sir Edward Downes, 85, travelled this summer to Dignitas to die together with his wife Joan, 74, who had terminal liver and pancreatic cancer, prompted questions over why he had been allowed to die too – when he was virtually blind and increasingly deaf, but not himself terminally ill. The same questions were asked when Daniel James, a 23-year-old rugby player, paralysed during a training accident, was helped to die.

Minelli offers dry cinnamon-and-nutmeg biscuits and an unusual Chinese tea – white monkey paw – which he has meticulously prepared, sticking a thermometer into the kettle, heating the water to precisely 70C, setting a digital alarm for five minutes to allow the tea to brew before decanting it into a vacuum flask. Then he sets out his vision like this: everyone should have the right to end their life, not just the terminally ill, but anyone who wants to, and he passes no moral judgment on their wishes. "We don't discuss moral questions. What moral? Which moral? Catholic? Muslim? Buddhist? We are just working of the atheist basis of self-determination," he says.

Section 115 of the Swiss Criminal Code says that anyone who acts on selfish motives to assist someone to kill themselves can be punished with up to five years in jail. The law has been interpreted by Dignitas and other assisted suicide organisations as meaning that assisted suicide is not illegal as long as there is no selfish intent (such as helping an aunt to die in order to get her inheritance).

But the Swiss medical regulations inhibit Minelli's more radical ideas, prohibiting doctors from prescribing drugs to healthy people, and restricting involvement in assisted suicide for the mentally ill – making it practically impossible for Dignitas to help people who are profoundly depressed to die. This is a prohibition that Minelli is fighting.

So far there have been no prosecutions following any of the suicides he has helped organise (for people from more than 60 countries, 132 from Britain) but Minelli is involved in a handful of legal battles with the Swiss government, determined to clarify the law which governs suicide.

"We have a lot of members [who have had] depression for years and years and years. They say, 'We have tried so many treatments and they haven't worked.' If they tell you 'I have been depressed for 15 years and I don't intend to be so for another 15 years', who should say no to that?" In extremis, he will offer advice on how to end one's life efficiently at home.

Breaking the taboo of suicide

Three firmly held beliefs lie beneath this practice. First, his conviction that once you give someone the freedom to talk about suicide this reduces their desire to go ahead with it. Second, he believes that even the offer, in the abstract, of an assisted suicide gives someone who is in pain a lot of relief – they know that their future no longer rests on a decision between enduring "the hell of their own suffering or attempting a high-risk suicide by themselves". His research shows that 80% of those who get the green light to go ahead with an assisted suicide do not go through with it.

Third, he argues that providing a service to help people kill themselves properly will reduce the large number of catastrophically failed suicides. He is appalled by the prevalence of botched suicides, committed in isolation by desperate people who do not have the expertise necessary to succeed. He points out that it is now very difficult to kill oneself by overdosing on tablets – instead they ruin the functioning of their liver. Jumping from a building, throwing oneself beneath a train, and trying to use a gun also tend not be very effective, he points out, frequently leaving the individual alive but in a terrible state physically. These failed suicide attempts end up putting a heavy burden on a nation's health service, he says, another motivation for his organisation's work.

"If we want to reduce the number of suicides and suicide attempts, we should break the taboo of suicide. We should not say suicide should not happen, we should say suicide is a marvellous opportunity given to man to withdraw them from a situation that is unbearable for them," he argues.

His fondness for describing suicide as a "marvellous opportunity" is very irritating to conservative Swiss officials who object to the country's new image as a suicide tourism destination. (Minelli brushes off the suggestion that his work has damaged the nation's reputation, with a typically acid aside: "Switzerland was already famous for tax-evasion tourism.")

Whilst in Britain the director of public prosecutions, Keir Starmer QC, is working on reducing the likelihood of being prosecuted for assisting a suicide, the trend in Switzerland is edging in the other direction. The Swiss government last month announced that it would consult on whether to ban, or call for greater regulation of, assisted suicide. On a more personal level, one of Minelli's opponents in the public prosecutor's office has told him that there will eventually be a "biological solution" to the problem of Dignitas, hinting that he hopes Minelli will drop dead.

Minelli courts controversy with some of his more inflammatory comments. Condemning the Swiss government's campaigns to regulate the arrival of suicidal foreigners, he remarks: "In the second world war they closed the borders to Jews and those Jews who wanted to come here were repelled, and were murdered in concentration camps. And now we have people looking to end their lives in Switzerland and they are sent back and forced to live on. What is the difference? What is more cruel?"

His decision to found Dignitas, leaving behind a career as a human rights lawyer, has its roots in a childhood memory of witnessing his dying grandmother begging her doctor in vain to help her end things. The experience inspired an attachment to the concept of a good death.

"Death is the end of our life. After a good life, we should have a good death. A good death is death without pain, where you can say 'I had a good life, and I can now go to the other side,'" he says. "Nowadays, death is exported to institutions, to hospitals. Death has become a lonely occasion."

To illustrate how a good death should take place, Minelli offers a visit to the apartment where Dignitas members can come to die. Cheerful and eager to be helpful, he arrives to collect me the following morning, dressed in sagging brown corduroy jacket, faded blue T-shirt, blue silk cravat and socks beneath his Velcro-strapped sandals. He has been up since 5.15am at his computer, and worked late the night before too, driving several miles to see whether a Greek restaurant owner might be persuaded to volunteer as an interpreter should the suicidal Greek woman return. Despite this, he is bouncing with energy, running up steps and striding around.

As we drive through the autumnal Swiss lakeside landscape, past silver birches with golden leaves, wooden chalets with neat green shutters and cascading red geraniums, he describes the multiple difficulties he has had in finding a permanent place to carry out the suicides. Neighbours at earlier apartments complained at the constant presence of undertakers, while another flat in a purely residential area was shut down by the local council. Permission to offer his own sitting room as a venue was refused. For a while, suicides were carried out in hotel rooms and a few people from Germany decided they would prefer to die in their own cars in a motorway lay-by.

A new flat in an industrial area was so brutal in its simplicity that several relatives were horrified by the surroundings and one, Daniel Gall, was so upset that he wrote a book denouncing the experience, published earlier this year, J'ai Accompagné Ma Soeur (I Accompanied My Sister). "Very ugly. Very, very ugly," Gall tells me over the phone. "It was the most horrible factory, next to the biggest brothel in Zurich. The conditions were monstrous." Minelli shrugs off the complaint lightly, retorting that someone accustomed to staying in five-star hotels would probably have been unimpressed by the earlier flat.

Finally, this summer, the two-storey house in Pfäffikon was bought for around €1m (£880,000) – much of it raised by donations from members. A newsletter sent out this month to members has pictures of the site, holiday-brochure style, with alluring captions: "Beside lies a tiny lake; a little waterfall dabbles."

After the Heidi-esque scenery we have driven through, the location of the modern, blue-metal construction is rather a surprise. The house is in an industrial zone, in the shadows of a vast grey machine-components factory; to the left there are factories, to the right there are factories, in front there is a football pitch. It's not that the place is exactly charmless, it is just a bit peculiar. To enter, guests make their way across wooden decking over a large goldfish pond (which does have a tinkling water feature), and then they arrive in a light, open-plan room, with a hospital bed (which reclines electronically) in one corner, and a large white sofa in another. There is another room with a second bed to die in across the hallway. By the bed there is a CD player and a few CDs – Offenbach's Gaîté Parisienne and Vivaldi's La Stravaganza – left by former clients. There are open boxes of tissues ready on the tables. The former owner had the constellation of Orion picked out in halogen lights in the ceiling. On the shelves there is a kitsch stone statue of a cherub, and a few slightly wilting orchids. There is nothing funereal about the place; instead the space is sunny, clean and neutral, not unlike a holiday rental apartment.

"We think that if you go to a location for your last moments, it should be adequate. It should be nice and dignified," Minelli says.

'They can go home any time'

People who travel to Switzerland to die with Dignitas are encouraged to come with family and friends, who stay with them as they drink the lethal dose; one person brought 12 friends with him. Dignitas staff are happy to give advice on good restaurants for a final meal, nearby cinemas and excursions to the mountains, for the preceding days, but they observe that usually members are keen just to get on with dying.

Staff suggest that everyone should arrive at the flat at 11am (that way the police formalities which happen after the death can take place during office hours, which keeps the local officials in good humour).

Minelli says he is never present at the deaths. Instead, Beatrice Bucher, a paid member of Dignitas staff who now works in the head office but has been a companion at more than 20 deaths, describes the process. She has a quietly compassionate tone, soothing and sympathetic, and believes strongly that she is performing an important role in society. "They need to know that they can go home at any time. I'm constantly asking if this is what they want. I have to be clear that this is the really the moment," she says. On more than one occasion she has helped people return home who have changed their mind. "One woman still calls me to say thank you," she says.

The first stage happens at a round table, covered with a yellow tablecloth, where the two Dignitas companions sit with family members and the individual who is about to die to discuss the procedure. At this stage, a lot of documents must be signed setting out the desire to die. It is up to the members to decide when they are ready to take an anti-vomiting drug to prepare the stomach, and half an hour later, the lethal drug. "I tell them, 'You are the boss. You must tell me when it is time for me to prepare the drugs,'" Bucher says.

"If someone wants to talk about their life for six hours, we will never hurry them," Minelli says. "The music, all the details, are their choice. We are servants of their desire for self-determination."

Bucher stays with the family and goes through the documents. "Sometimes they will sit at the table and talk about their family and their life and we have a nice time. Sometimes the person who is going to die will appear to be angry and quite bossy, and tell me to hurry up, but I know it is not how they are feeling inside," she says.

She has to judge when the time is right for both the person who wants to die, and their relatives. "Once I had a mother – not so old, in her 50s – who was really ill. She came with her daughter who was perhaps 25. The mother was very firm that she would go quickly and that it was not a problem. She told the daughter that she was not to cry and made her go and stand in the kitchen. I had to explain that this is not the way, you should not tell your daughter she cannot cry," she says. Staff also suggest that relatives stay to witness the death, because they believe this helps with the mourning process.

People are encouraged to lie down, because if they die sitting up at the table, their mouth drops open and their body slumps, and it is harder for the family to watch the process. "Then we install the film in the video camera, but I am always asking 'Do you need more time?' Usually they are calm. Most of them are in a lot of pain and they know that this drink will end it forever."

The 15g of white powder is mixed with water and drunk from a small glass. Bucher advises people to say anything they need to say, their final words, before they drink, because after there is not much time – usually just between one to three minutes before they sleep, fall into a coma and then die. "Some people say thank you and tell their family they love them, that they have had a really good life and that they are grateful that they can die," she says.

She warns them that the drink will be bitter, and some people choose to neutralise the taste with a chocolate. "They feel good. There is no pain. It's like before an operation – they feel woozy," she says.

"Another time, there was a mother who clearly did not have a good relationship with her two daughters who were with her. It was very strained. But after she drank, she took them in her arms and said 'I love you, you are my best ones,'" Bucher says, still moved by the memory. "Then she died. They said it was the first time she had hugged them like that. That was a good moment for me – it was not too late for her to show how she felt."

As soon as the person dies, the undertakers and police are called. In a side room, there is a television for the police to watch the video, so they can file a report. Upstairs, there is a washing machine, and a box with some folded clothes and shoes belonging to recently dead people, ready to be dispatched to the Red Cross.

Minelli has delegated much of the organisation of Dignitas to his staff of 10 part-time workers. The Dignitas office, in a street near his home, 20 minutes drive from the Pfäffikon apartment, is very office-like – no sofas or handkerchiefs. He checks the files, and notes that one English person is booked in to die this week, but otherwise there is an unexpected lull in appointments. Bucher puts it down to the Indian summer most of Europe has experienced, and predicts that things will get a bit busier in the run up to Christmas.

"We have had good weather for the last few weeks, so people don't call us so much," she says.

Minelli meets people here occasionally to discuss their desire to die, but mostly his work is concentrated on the court cases and campaigning. Back at his house, where he lives alone, he describes with enthusiasm a new technique for painless death he is experimenting with; one which uses a chemical that is easily available without the need for a doctor's prescription. He requests that we do not publish details of the chemical or the technique, to prevent it becoming more widely used. The method can be administered easily by staff, and using this he could circumvent using doctors altogether. He struggles with hanging on to doctors, just as he struggles with keeping apartments; most are nervous about co-operating with Dignitas for fear of losing their licence.

Costs from the various legal battles cost around £100,000 every year, money which is raised through the annual membership fee and periodic appeals to supporters for funds. Minelli says he does not pay himself a salary, and remarks, "I have made a lot of debt in order to maintain Dignitas."

An estranged colleague, Soraya Wernli, who worked for several years helping with the suicides, lost faith in the organisation and told the police around five years ago that Minelli was making money from death and the fear of it, and criticised him for running "a production line concerned only with profits". Police investigations found nothing suspicious.

Minelli's novelist daughter Michele, who has arrived to visit her father, remarks that she and her sister will have no inheritance when her father dies because everything has been spent on his campaigning work. She was wounded by Wernli's allegations, more sensitive to criticism of her father than he is on his own behalf. ("He doesn't mind people throwing tomatoes at him," she says.) Disturbed by the claims, she offered to help him gather feedback from the relatives of people who have died, and now she is responsible for sending out forms and compiling responses. The overwhelmingly positive replies have reassured her, and she collects a few from the pile of new post and spreads them out over the worn red-checked tablecloth.

Attempts to dissuade applicants

One person, from Britain who recently came to witness a relative's death, describes the process as a "calm day filled with the deepest sorrow I have ever felt", before thanking Dignitas for its assistance. Another person who also travelled earlier in the autumn from Britain says the experience was "a time of sadness, naturally, but also of peace, calmness, spiritual comfort in a relaxed, compassionate, unhurried atmosphere". "Long may you continue your good work," another writes.

The doorbell rings again and it is the Greek woman back again with her uncle and a translator who she has managed to find somewhere in the city. This time, Minelli invites her in; they sit in the main room out of sight but her anguished voice can be heard clearly. "Mr Minelli! Mr Minelli! Mr Minelli!" she keeps interrupting him, angry, as he tries to explain that she needs to bring him a complete medical history before her case can be considered.

As it becomes clear that he will not help her to die, she begins shouting: "Ach, Mr Minelli! Ach, Mr Minelli!" He remains calm, explaining once again that she must come fully equipped with her medical records so that a doctor can consider whether to prescribe a drug. After almost an hour or so they leave, promising to return from Greece with more documents in the spring. Minelli explains that she suffers from paranoid schizophrenia and is determined to die. Whether he will be able to help depends on whether a Greek psychiatrist can write a letter that says she is capable of rational thought. He is despondent at the desperate steps that people are forced to take in their search for a painless death, steps which he compares with the measures women once had to take if they wanted an abortion.

He hopes that she will reconsider, and happily recounts stories of other applicants who have been persuaded to change their minds. When the depressed young German man arrived on his doorstep some years ago, demanding to die immediately, Minelli felt sorry for him, took him in, and spent a day or so explaining why suicide was not the answer. On the third morning, when the young man said once again that he wanted to die, Minelli took a new approach, telling him: "If you really want to die, there are three options. There is hanging, but it is very risky: if you are found too early you will live on, but as an idiot because the blood will have stopped flowing to your brain. You can go to the Swiss glacier, wearing light clothes, and you will die of cold, but if you are found too early you will lose your legs. Or you can stop eating and just drink tea and water."

"He said 'Yahoo! I will die by starvation.' He was completely happy. It was a 180-degree change," Minelli says. They drove together to a bathing resort 30km away, and they spent the afternoon swimming together.

"We came back here at midnight and looked through my telescope up at Jupiter with its four Gallilean moons and Saturn. He was delighted. We discussed cosmology and astronomy and I sent him to bed." The man went home to Germany, where Minelli put him in touch with a psychiatrist. His crisis passed and the two remain in occasional contact.

"As an amateur of astronomy, I know life is a speciality that is known only on earth and is something that is very rare and so we have to care as much as we can for life," Minelli says. "But we must also accept that a feeling human being must have the opportunity to say: This has been it. I have had now enough and I will now stop."