World leaders attending a UN summit in Rio on Wednesday weighed steps to root out poverty and protect the environment as thousands of activists marched through the city center in protest.

UN chief Ban Ki-moon warned that "time is not on our side" for fixing a mounting list of problems as he formally opened the UN summit on sustainable development.

The high-profile Rio+20 event, attended by 191 UN members, including 86 presidents and heads of government, comes 20 years after Rio's first Earth Summit when nations vowed to roll back climate change, desertification and species loss.

But thousands of activists attending a counter-summit staged a good-natured and colorful protest in central Rio to denounce Amazon rainforest deforestation, the plight of indigenous peoples and the "green economy" being advocated at the UN gathering.

The march drew environmentalists, workers, civil servants, black militants, homosexuals, indigenous peoples and feminists.

Organizers said 50,000 people turned up but police estimate the crowd at no more than 20,000.

At the summit, Maldives President Mohamed Waheed announced that his Indian Ocean archipelago planned to set up the world's biggest marine reserve to protect its fisheries and biodiversity.

He said the Maldives would become "the single largest marine reserve in the world," where only sustainable and eco-friendly fishing will be allowed.

A total of 191 speakers were to take the floor until Friday when the summit leaders are to give their seal of approval to a 53-page draft document agreed on by their negotiators Tuesday.

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.

In his opening remarks, the UN secretary general praised Brazil, the summit host, for securing a deal on the summit's final draft statement.

"The world is watching to see if words will translate into action as we know they must... It's time for all of us to think globally and long term, beginning here now in Rio, for time is not on our side," he said.

French President Francois Hollande described the deal on the draft as "a step" but "an insufficient step".

"It will be up to world leaders to make a positive step," he told a press conference.

"We recognize that the old model for economic development and social advancement is broken," Ban said later at a ceremonial event.

"Rio+20 has given us a unique chance to set it right... to set a new course that truly balances the imperatives of robust growth and economic development with the social and environmental dimensions of sustainable prosperity and human well-being," the UN chief said.

Brazilian President Dilma Rousseff, who was elected president of the conference, said she had no doubt "that we will be up to the challenges that the global situation imposes on us."

From the International Space Station, US, Russian and European astronauts sent greetings to the summit leaders, courtesy of the US space agency NASA.

As the summit got under way, eight multilateral development banks announced that they would set aside $175 billion to finance sustainable transport systems over the next decade.

The pledge was made jointly by the World Bank, Asian Development Bank, African Development Bank, Inter-American Development Bank, CAF-Development Bank of Latin America, European Bank for Reconstruction and Development, European Investment Bank and Islamic Development Bank.

Some of the most contentious issues discussed at the 10-day UN conference 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.

Environmentalists were scathing in their criticism of the summit, with Greenpeace calling it "an epic failure" while WWF said it was "significantly disappointing."

And a London-based NGO said it had put the Amazon rain forest up for sale on eBay -- starting price 99 pence (about $1.50) -- in protest at what it said was the British government's obsession with putting a financial price on the ecosystem.

"The UK government is promoting the sale of nature to the highest bidder. We set up the Rio+20 nature sale on eBay to demonstrate how ridiculous this is," said Kirsty Wright of the World Development Movement.

The bogus offer was removed by the online auction site, the group said.

Costa Rican President Laura Chinchilla meanwhile criticized the absence of some heavy hitters such as US President Barack Obama, British Prime Minister David Cameron and German Chancellor Angela Merkel.