World leaders open a UN summit Wednesday where they are set to endorse a blueprint for eradicating poverty and protecting the environment that critics insist is a threadbare compromise.
Some 92 leaders, including the host, Brazilian President Dilma Rousseff, and UN chief Ban Ki-moon are attending the Rio+20 summit on sustainable development.
The high-profile event comes 20 years after Rio's first Earth Summit, when nations vowed to roll back climate change, desertification and species loss.
The meeting is set to get under way at 10:00 am (1300 GMT) with the screening of a short film titled "State of the Planet" and a statement by the UN secretary general.
Some 191 speakers are expected to take the floor until Friday, when leaders will close the 10-day UN conference by giving their seal of approval to a 53-page document agreed by their negotiators Tuesday.
Not everyone was upbeat about the hard-fought draft.
"Nobody in that room adopting the text was happy. That's how weak it is. And they all knew," the European Union's commissioner for climate change, Connie Hedegaard, said on the micro-blogging website Twitter.
But US climate change envoy Todd Stern said the deal was "a good strong step forward" and that the text was unlikely to be altered.
As summit host, Brazil was keen to avoid pressing leaders too much over the final text, after the 2009 Copenhagen Summit nearly collapsed and was followed by furious exchanges among participating countries.
The draft outlines measures for tackling the planet's many environmental ills and lifting billions out of poverty through policies that nurture rather than squander natural resources.
Some of the most contentious issues were proposed measures to promote a green economy and the "Sustainable Development Goals" that are set to replace the United Nations' Millennium Development Goals after they expire in 2015.
Environmentalist groups were scathing in their criticism of the text.
"We were offered a common vision of inaction and destruction," Daniel Mittler, political director of Greenpeace International, told AFP.
"There's absolutely nothing there for people and the planet," he added.
Lasse Gustavsson of the World Wildlife Fund agreed, saying: "This is significantly disappointing. The language is very weak and the outcome of this conference will not be anywhere near what the people and the planet needs."
The leaders, including French President Francois Hollande, South African President Jacob Zuma, Prime Ministers Manmohan Singh of India and Wen Jiabao of China, will hear a message from astronauts in International Space Station on Wednesday.
But US President Barack Obama, British Prime Minister David Cameron and German Chancellor Angela Merkel will be absent.
And Russia will be represented by its head of government, Dmitry Medvedev, not President Vladimir Putin as expected, according to the list of speakers.
Ahead of the summit, 50,000 activists, business executives and policy-makers attended the 10-day UN conference.
On Wednesday, tens of thousands of leftist activists attending a Rio+20 counter-summit are to march through central Rio to rail again against what they see as capitalist attempts to hijack the "green economy."
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