JOHANNESBURG, Aug. 25— The smoke settles over the rickety shacks and shabby houses as soon as this city wakes. Thousands of poor people without electricity burn scraps of wood in rusty tin cans to keep warm. Others burn coal in old stoves that belch soot and fumes into the cold morning air.

Poverty in crowded cities like this one and in sleepy villages as well is threatening the air, the waters and the forests of the developing world. On Monday, the United Nations' World Summit on Sustainable Development will open here to try to focus the world's attention on the environment in these poor countries.

The 10-day meeting is expected to attract more than 100 presidents and prime ministers from Africa, Europe, Asia and Latin America, who will devise a plan to protect the globe's atmosphere, lakes, forests and wildlife and focus on the link between poverty and environmental degradation. Officials hope to build on the ambitious, but poorly executed, agenda set at the Earth Summit in Rio de Janeiro 10 years ago.

Leaders from the United States, Europe and developing nations have already agreed that reducing poverty must be a central element of the plan. But the question of how to do that and to ensure the survival of the globe's natural resources has left rich and poor nations bitterly divided. The dispute is likely to dominate the political negotiations here. In this continent of immense natural beauty and desperate poverty, the debate could hardly be more relevant.

In the 1990's, Africa had the world's highest rate of deforestation as poor people cleared trees for farmland and firewood. Acute respiratory infections, which often afflict families that rely on coal or firewood, kill or disable about 30 percent of sub-Saharan Africa's children each year, the United Nations says. Meanwhile, pollution is worsening as millions of Africans abandon rural villages for urban shantytowns.

Poor countries say they cannot safeguard their natural resources unless they can strengthen their economies. They want wealthy nations to commit 0.7 percent of their gross national product to aid developing countries; to reduce or eliminate tariffs on agricultural goods from poor countries and to halve the number of people without access to sanitation by 2015.

Some wealthy nations, including the United States and some members of the European Union, are resisting. American officials say they have already agreed to increase foreign aid to the poor, and developing nations should eliminate corruption and strengthen democratic institutions before more aid is committed.

The United States, the world's biggest polluter, has also refused to commit to time frames for reducing greenhouse gas emissions or for converting to renewable energy sources, despite pressure from the developing world and the European Union.

The European Union has agreed to discuss targets for increasing foreign assistance, but opposes time frames for eliminating agricultural subsidies, which protect their farmers from foreign competition.

Environmentalists and advocates for the poor, who have poured into this city by the thousands, have already been holding marches to keep the link between poverty and environmental decay high on the agenda.

Much of their anger is directed at President Bush, who will not attend.

''The north/south dispute over money and trade is an old one, but it's particularly acute at the moment,'' Mark Malloch Brown, administrator of the United Nations Development Program, said in a telephone interview from New York. ''There's distrust on both sides.''

Meeting organizers say they believe that the differences will be resolved. They note that the nations have already agreed on nearly 80 percent of the summit's action plan.

They have already agreed to offer incentives for investment in cleaner forms of production, to provide additional resources to keep deserts from spreading and to try to reduce by half the number of people living on less than $1 a day by 2015. But none of these commitments are groundbreaking; the commitment on poverty, for instance, was adopted two years ago at the Millennium Summit at the United Nations.

Officials assembled here agree that developing countries must focus on good governance and democracy, and curb pollution within their own borders. But the South African president, Thabo Mbeki, who has led Africa's push for more foreign aid, has said leaders should also take a stand for the environment by committing to alleviating poverty.

''You can't expect the developing countries to address the environment in the absence of economic growth and development,'' said Crispian Olver, director general of South Africa's department of environmental affairs.

The meeting in Johannesburg will be the third international effort in 30 years to find a way to promote human development without damaging the environment. But countries have been slow to act.

After the first meeting, in Stockholm in 1972, wealthy nations began cleaning air and water, but continued to ravage forests and other resources elsewhere to maintain growth.

In Rio, at the second meeting, leaders signed an ambitious agenda to protect the environment while strengthening the economies of poor countries. But there were few binding obligations.

Since then, the world's population has continued to surge, poverty has deepened in Africa, forests have retreated, fish stocks have decreased and concentrations of greenhouse gases have increased.