Calcutta is unique among Indian cities. Not for its poverty and destitution, although it has become world famous for that. It is unique because alone of all the cities in the vast Indian sub-continent it has an underground railway.

There is only one line about 10 miles long with 16 stations. It straddles the port city and it took more than 15 years to build by hand. But Calcuttans and outsiders will tell you that it is the one place in the city that is spotless. People have been fined and jailed for dropping litter in what became a showpiece for the capital of West Bengal when it opened.

Now it is shabbier and jammed morning and evening with commuters, but it works and it is clean in a city that looks as if it is slowly crumbling.

For Calcutta is a long way from its halcyon days of grandeur when it was the capital of British India and a small city of fine palaces and splendid boulevards. It was the world's tea-trading capital and an important port of call for ships which moved up the city's mighty Hoogly river. It is still a city of culture and the arts and was home to the Nobel prize-winning poet Rabindranath Tagore and, much later, the film-maker Satyajit Ray.

But it is now a city of 14 million, 40 per cent of whom live in slums or on the streets, and in a country where the population increases annually by 20 million, four times the population of Ireland.

Calcutta's own population swelled in the early 1970s when Pakistan went to war with India and the former state of East Pakistan became Bangladesh. Millions of refugees fled across the border into Bengal and made their way to the capital.

Even before then Mother Teresa was in action. The diminutive nun, less than five feet tall, had left the security of the Loreto Convent in Calcutta in 1948 after 15 years working and teaching.

Sister Marie Therese Breen, an 89-year-old nun originally from Fairview in Dublin, was a Loreto novice with the then Sister Teresa. Her first recollection of her is as an "ordinary simple girl who was very nice to talk to". There was nothing strikingly different then about Mother Teresa, who was born Agnes Gonxha Bojaxhiu in Skopje, now the capital of Macedonia.

"We had no idea of what she was going to do. We got the surprise of our lives when she left," says Sister Marie Therese who adds that Mother Teresa had always talked about the poor and had gone to visit them. "She will be greatly missed and she was a living saint because of what she did."

The nun everybody calls simply Ma or Mother started an order which today has 4,500 nuns, priests and brothers in 110 countries. She is the icon for poverty and became known as the saint of the gutter for her devotion and commitment to the poor, destitute and dying.

Although she began her work in 1948, it was only in the 1970s after the film by Malcolm Muggeridge, Something Beautiful for God, that Mother Teresa came to world attention. Since then the city has been a stopping-off point for every visiting dignitary, from princess to president.

In the week after her death the world's church and government leaders have come to pay respects to the woman who attracted larger crowds and greater respect than any politician.

Yesterday morning, outside one of her 14 institutions, hundreds had gathered. A Bengali actress was visiting the Nirmal Hriday home for the dying in Kalighat, a poor area in the south-west of the city. The poverty is obvious, with open sewers, stagnant pools of water after torrential monsoon rains, and people trying to wash themselves from taps on the street.

Police had to be called to push away the crowds waiting for a glimpse of the glamorous film star. The home was previously a temple and was handed over to Mother Teresa and her nuns to operate.

Inside the scene is stark and tragic. There are 110 patients, in two wards, one for men and one for women. In each there are three rows of up to 15 beds, and patients sit or lie in varying degrees of illness. The high-ceiling ward is clean and the patients have clean clothes.

Some patients are relatively healthy, but some are so emaciated they seem on the point of death. One man who slips in and out of consciousness is being fed by Raphael, a 30-year-old industrial designer from Barcelona, in Spain. "I do not know his name but this man was brought in 10 days ago. Sometimes he recognises me, sometimes he doesn't."

The nuns, nurses and volunteers know he will die in a few days, but, said Sister Dolores who is in charge of the home, "he will have some tender love and care for those few days".

There are seven sisters working full-time in the wards. "We are not a hospital and we don't diagnose, but we know. Some are spitting blood and have TB. They might have chronic asthma or malaria or wounds with maggots in them."

Dipko Sarkar is a 38-year-old rickshaw wallah who lived on the street. He runs on foot pulling a two-wheeled carriage which earns him 50 to 60 rupees a day, about £1. He is suffering from malnutrition. "I had no one to look after me and was not making enough money. So I came here 17 days ago," he says through an interpreter. "I hope to get better and to be able to go back out and work again."

The man in the next bed is Santosh who worked in a restaurant but burned his leg badly with hot oil and lost his job. Both men are Hindus.

In the women's ward, Lucy sits with a serene smile. She has been in the home for 11 years and cannot walk because of chronic rheumatism. "I was orphaned very young and have been looked after by the sisters since," she says. She has a bed in the women's ward and does not go out. Although she can read and write she has nothing to read. "I accept this," she says. She was one of the mourners following the funeral procession for Mother Teresa early today.

Throughout the week, thousands of people were gathering for the funeral. The city ran out of flowers. They had to be imported from outside to cope with the demand from the population to pay its respects.

It is relief work at its most basic, and that is a major criticism of the work Mother Teresa's order does. But her nuns, including Sister Nirula Maria, originally from Letterkenny, Co Donegal, points out that Mother never set herself up to be a social reformer. Her mission was to feed the hungry and clothe the poor. It was for society to take up this role.

Our interpreter, Anthony Nelson, is manager in one of the city's smaller hotels. He does a few days' work volunteering but brings visitors to see the homes and, in the past, to meet Mother.

"A lot of business people gave money to Mother Teresa for her homes", he says. And will they give now that she is gone? "That is a very good question. I think that they will, but whether they will give as much is hard to say. They trusted Mother in part because she was a foreigner."

Mr Nelson mentions the homes for those with leprosy. "Many people give a lot of money, but they will not go to those homes. They are afraid because they fear for themselves or their children."

He added that this was what people really admired Mother Teresa for. "She went and touched and hugged lepers and she worked with and gave food and medicine to the untouchables in society."

Sohinder Grewar clutches her chest as she talks about what Mother Teresa used to do. "We would find it hard to hold down our food when she would pick up and hug someone to her, someone no one else would touch."

Sitting in her apartment home, Ms Grewar talks of her work as a volunteer with Mother Teresa with mentally handicapped people. They were called NCLs or non-criminal lunatics.

When they were abandoned by their families there was nowhere to put them, so they went to jail, even though they had done nothing wrong. Mother Teresa set up homes for them, says Ms Grewar, who helped by teaching them to embroider cross-stitch. It takes a day to do a small design which is used for greeting cards that sell for 20 rupees each. The girls will never be independent, but some have been taken back by their families and, because of their contribution of 200 rupees a month, a younger child in the family can be educated.

Now that Mother Teresa is gone the question many ask is how the order will survive her. Some say it is in chaos. Sister Anne-Therese, a contemplative member, says that "an order is at its peak with its founder. Things do not remain static and there is always change. With prayer and dedication and Mother watching over us we will continue."

Father Joe Langford is the American co-founder with Mother Teresa of the priests' branch of the Missionaries of Charity. Started in 1983 there are now 14 priests and 50 seminarians based in Mexico. Quoting Mother Teresa, he said she described herself as "a small pencil in the hand of God by which he used to write a love letter to the world. In the last week the pencil has disappeared but the love letter lives on."