LONDON -- South of the river, in a slightly seedy section of the city, in a down-at-the-heels building that has seen better days but not better uses, the Anti-Slavery Society is still in business, because slavery is too. Founded in 1839, it is the world's oldest human rights organization.

As it raises its small voice amid the incessant clamor for public attention, it faces the problem of stark incredulity concerning its threshold assertion: Slavery exists.

Today there may be as many, perhaps many more, people in conditions properly denoted as slavery as there were when the society was founded. So says the society's director, Lesley Roberts.

She is not one of those flurried people one so often sees in the world of altruism, the kind who perpetually look as though they had risen late and dressed in haste. She has the brisk manner of one who has worked in banks, which she has done, and she has the restlessness to have been bored by that, which she was. In an office lit only, and dimly, by watery sunlight that struggles through unwashed windows, she enumerates the society's concerns, beginning with chattel slavery -- the total ownership of one person by another. It was officially outlawed in the Arabian Peninsula less than 20 years ago and may even be expanding in parts of northern Africa, particularly Sudan.

Far more prevalent, and properly called slavery, is debt bondage, especially of children. Such laborers are bound for payment of debts and are subservient to a power from which there is no appeal.

There are workers in servitude in both hemispheres. Roberts says that many Haitians living illegally in the Dominican Republic have been, in effect, sold to state-owned sugar plantations. But the biggest problems are in Asia, where slavery often is a particularly odious, because routinized, form of child abuse.

Children are, in a sense, ''natural'' slaves -- a renewable source of slaves -- because they are born into total dependence on adults. Children are sometimes sold outright, with common prices ranging from $20 in the Sudan to $200 in Bangkok.

Social traditions no longer sanctioned by laws -- traditions of caste and class -- can still be as strong as iron fetters. There is the tradition of giving a child for, say, a year in payment of debt -- sometimes a debt incurred by parents who borrow from loan sharks, using children as collateral. At the end of the year, the person holding the child in bondage may say the child has eaten so much that the bondage must be extended.

No one knows even within several scores of millions how many children are enslaved in domestic service in sweatshops or in Asia's huge sex trade. When computing the probable numbers, says Roberts, ''it is not hard to get up to 200 million.''

In India, for example, the government says there are fewer than 200,000 bonded laborers. The society's sources say 5 million. Look down, dear reader, you may be walking on them.

Demand for Indian carpets soared when the price of Iranian carpets rose after the shah outlawed child labor in the early 1970s. Today in India's main carpet-weaving region (small hands, such as those of 7-year-olds, tie especially tight knots), there are 100,000 malnourished children employed. Some 15 percent of them, according to the society, were sold into bondage in spite of the fact that India outlawed debt bondage 14 years ago.

They often work 12-hour days (poorly fed; subjected to corporal punishment; often sleeping at their looms six, sometimes seven, days a week for a weekly wage sometimes as low as $1). Today, as two centuries ago, slavery pays. The labor costs of a carpet that may sell in London for $6,000 can be as little as $20.

While sifting abundant evidence of such slavery (and worse: millions of children, male and female, are devoured by Asia's prostitution industry), the society maintains a remarkable emotional equipoise, advocating piecemeal remedies for absolute evils. For example, it does not favor banning imports of carpets made with coerced labor. It knows that destroying the carpet industry would destroy many lives. An existing industry can be improved.

Publicity, embarrassment, persuasion, organized labor -- these are among the remedies required. But the first requirement is the patience of politics, which acquires special dignity in the face of such shattering facts.

Roberts and the society (180 Brixton Rd., SW9 6AT) are echoes of the distinctive moral earnestness that redeemed the 19th century and leaven this one. ''I get bored easily,'' she says. ''This is not boring.''