Around a hundred heads of state or government are expected to attend the three-day meeting of 193 nations, the 20-year follow-up to the United Nations' Earth Summit.

But negotiations over a master plan to be issued on Friday ran into problems and activists braced for what they feared would be a sad compromise.

On the table in Rio is a 50-page draft that would identify the world's many ills and spell out how the community of nations plans to treat them.

Called "The Future We Want," the communique is scheduled to be endorsed on Friday, setting down guidelines for sustainable development for the coming decade and beyond.

Months of work have been invested in the project.

Nations and regional blocs have haggled over how to promote the green economy, muster funds to help cleaner growth in poor countries and define "Sustainable Development Goals" which would succeed the UN's Millennium Development Goals after they expire in 2015.

The outcome of the talks remained unresolved early Tuesday, as Europe complained commitments were not strong enough.

Brice Lalonde, a former French environment minister who is the conference's co-coordinator, said he believed the Sustainable Development Goals could emerge as the star of the event.

"The goals will affect rich countries and poor alike," he said in an interview with AFP on Monday.

"It may launch a new phase in the history of the United Nations. Instead of there being a difficult dialogue between developing and developed countries, between recipients on one side and donors on the other, we will have a discussion among the family of nations, facing mutual problems which it seeks to resolve mutually."

But green activists and campaigners for poverty eradication said the text was already unambitious before negotiations stepped up a gear last Wednesday, and some predicted a thick serving of fudge was on the menu.

In a message to the conference, 40 figures, including former heads of state and Nobel laureates, said the scientific evidence of dangerous environmental overreach is "unequivocal."

"We are on the threshold of a future with unprecedented environmental risks," they said.

"The combined effects of climate change, resource scarcity, loss of biodiversity and ecosystem resilience at a time of increased demand poses a real threat to humanity's welfare.

"Such a future generates unacceptable risks that will undermine the resilience of the planet and its inhabitants."

Signatories included Nobel chemistry laureate Yuan-Tseh Lee, Brazilian Environment Minister Izabella Monica Vieira Teixeira and Gro Harlem Brundtland, the former Norwegian prime minister who in the 1980s issued a landmark report on sustainable development.

The Conference on Sustainable Development is the 20-year follow-up to the Rio Earth Summit, where UN members launched offensives to roll back climate change, desertification, species loss and poverty.

The summit will be attended by President Francois Hollande of France and President Jacob Zuma of South Africa alongside prime ministers Manmohan Singh of India and Wen Jiabao of China.

But there will be conspicuous absentees, including US President Barack Obama, who will be represented by US Secretary of State Hillary Clinton, while environment ministers will stand in for British Prime Minister David Cameron and German Chancellor Angela Merkel.

Around 50,000 activists, business executives and policy-makers are attending the 10-day forum.

Many experts believe side events are far more effective in practical terms than the political declaration expected on Friday.

A "counter-summit" is being held in central Rio, some 40 kilometers (25 miles) from the sprawling convention center, which has been declared United Nations territory for the occasion