JOHANNESBURG -- The UN earth summit opened in Johannesburg Monday with South African President Thabo Mbeki warning that a widening gap between rich and poor was a "river" dividing the world, AFP reported.

Politicians from around the world face 10 days of intense negotiations in Johannesburg to overcome rifts between rich and poor countries and between the United States and Europe on alleviating poverty worldwide while protecting the environment.

"Poverty, underdevelopment, inequality within and among countries, together with the worsening global ecological crisis, sum up the dark shadow under which most of the world lives," Mbeki said.

But he said that grim picture could be changed.

"For the first time in human history, human society possesses the capacity, the knowledge and the resources to eradicate poverty and underdevelopment," he said, calling for the summit to produce "credible and meaningful" results.

More than 4,000 government delegates are taking part, 3,000 non-governmental delegates are observing the discussions, and 2,000 journalists are reporting them. Added to that are around 20,000 people from corporations and non-governmental organizations, attending workshops and other meetings.

This summit, officially known as the UN World Summit on Sustainable Development, is a follow-up to the first earth summit in Rio de Janeiro in 1992, which put forward 2,500 recommendations, most of them ignored.

Diplomats from more than 30 key countries met behind closed doors in Johannesburg over the weekend in a bid to bridge the differences.

South African Foreign Minister Nkosazana Dlamini-Zuma told journalists they had made some progress, but gave no details.

Sticking points include market access for developing nations, subsidies to farmers in the United States and Europe, and timetables for action.

A major disagreement is over objectives for poverty relief, which the European Union says are indispensible but which the United States refuses to endorse, in line with its reluctance to enter into any new multilateral deals.

"We don't see a need for new targets," John Turner, U.S. assistant secretary of state for international environmental affairs, told journalists.

He said however there had been "very substantial progress" on international trade and finance for development, two of the most politically explosive areas of the conference.

Thousands of activists have also descended on Johannesburg to promote their causes, with the most extreme vowing to disrupt the summit.

Dlamini-Zuma warned them that South Africa would not allow "anarchy".

Police fired three stun grenades without warning into an unauthorized march in Johannesburg by some 500 anti-globalization protestors.

On Wednesday and Thursday, police in Johannesburg arrested more than 100 landless demonstrators -- South Africans protesting against evictions from squatter camps -- and on Saturday they arrested 12 Greenpeace commandos after six of them scaled South Africa's nuclear power plant near Cape Town to unfurl anti-nuclear banners.

Some 8,000 additional police officers have been brought into Johannesburg to secure the city during the 10 days of talks, police spokeswoman Henriette Bester told AFP.

Security services are using an unmanned plane to watch for troublemakers and any attempt to ambush the motorcades of the 104 heads of state who have said they will attend the last two or three days of the 10-day conference. Combat helicopters are on standby, and plans are in place to protect delegates from snipers, air attacks and mortar fire.

Authorities have authorized eight marches during the summit, with the biggest a march of 10,000 landlesss people set for next Saturday.