RIO DE JANEIRO -- Indigenous people with painted faces and plumes have competed in the “Green Games” to highlight the need to preserve the natural world, a mock favela has been built on the beach as a reminder not to forget the poor and the Greenpeace ship Rainbow Warrior has pulled into Guanabara Bay.

And all of that has taken place before the main event — three days of high-level talks among world leaders that begin Wednesday at the United Nations Conference on Sustainable Development, also known as Rio+20 because it marks the 20th anniversary of the 1992 Earth Summit in Rio de Janeiro.

Some 50,000 people — an expected 120 heads of state or government, environmentalists, business leaders, social activists, artists, and protestors — have gathered for this global extravaganza on the future of the planet.

It’s the largest event in U.N. history and to keep the delegates and activists safe, Brazil has deployed 15,000 members of the armed forces and police and a Navy frigate is patrolling the bay. Some 8,000 members of the military will be charged with guarding heads of state, as well as Riocentro — where the high-level event takes place — airports, hotels and other venues.

World leaders began arriving Tuesday, but there will be some notable absences, including President Barack Obama, British Prime Minister David Cameron, and German Chancellor Angela Merkel. Secretary of State Hillary Clinton is leading the U.S. delegation. Among those who will be in attendance is Iranian President Mahmoud Ahmadinejad.

The challenge for delegates is to decide how to encourage a green economy while balancing the need for development with social welfare demands.

“Our current economic model is enormously inefficient in the way it does or doesn’t use resources,’’ says Rachel Kyte, World Bank vice president for sustainable development. “We need a different kind of growth, a greener and more inclusive growth.’’

There is scarcely an environmental or development topic that won’t get an airing by the time the U.N. conference and side events wrap up Friday: destruction of the rain forest, vanishing coral reefs, land grabs, the need for food security, clean water, the role of women in food production, oil exploration, safe drinking water, energy access, clogged transit systems, jobs, and sustainable development as a way to fight poverty and as an answer to the global economic crisis.

In an effort to be inclusive, the United Nations Development Program launched a website (www.vote.riodialogues.com) where topics that will be submitted to world leaders were debated and voted on by the international community. Voting by the public winnowed the list of 100 recommendations to 30.

Meanwhile, after six days of negotiations and consultations, work was finalized Tuesday on a 49-page document that leaders will debate and be asked to approve during their sessions. The document, which includes a process to establish a high-level forum on sustainable development, could define the agenda for decades to come.

Parallel and protest gatherings began last week, including the Jogos Verdes (Green Games) that pitted more than 1,000 indigenous athletes against each other in events such as archery and spear throwing. Unlike the 2016 Olympic Games, which will be held in Rio, the purpose of these games was to call attention to nature and preserving the environment.

The Brazilian government is sponsoring one of the main sideline events, the People’s Summit. In an atmosphere somewhat reminiscent of Woodstock, activities have played out along Copacabana beach where a replica of the Statue of Liberty with smoke belching from its torch and a plaque that reads “Freedom to pollute’’ has been built. There’s also a faux favela — a reminder to keep the poor in focus during conference debates.

“You have the poor living in areas that are very polluted, like the favela of Mandela,’’ says Pastor Antônio Carlos Costa, president of Rio de Paz, a human rights and protest movement. The Mandela favela recently made headlines when pictures of kids swimming in a river contaminated with fecal matter were published.

So maxed out are the city’s hotels that some 15,000 people are sleeping on cushions in public schools, Flamengo Park and at the Sambódromo, where the city’s famed samba parades take place during Carnaval.

Brazil itself is a microcosm of the tensions and contradictions in trying to forge a viable sustainable development policy and the tug of war between progress and environmental protection.

To its credit, it has moved 40 million people into the middleclass in less than a decade and Rio is trying to tackle public transportation problems with a subway extension and a newly inaugurated Bus Rapid Transit line.

But one of the most beautiful cities in the world is marred by polluted Guanabara Bay. Oranges, pieces of plastic and other debris floated in the bay near the berth of the Greenpeace Rainbow Warrior last week. Greenpeace has been an ardent foe of Brazil’s plan to develop its pre-salt deepwater oil fields. Greenpeace says consumption of oil from the pre-salt layer will result in 35 billion tons of additional CO2 in the atmosphere over 40 years.

While Brazilian President Dilma Rousseff has called on all countries to commit to an accord in Rio that addresses the most pressing global environmental and social problems, she has taken some criticism at home.

“In the Dilma government, we are seeing the largest environmental setbacks,’’ says Pedro Ivo Batista, an aide to Marina Silva, a former environmental minister who ran against Rousseff for the presidency.

Perhaps the single issue that has provoked the most ire among Brazilian activists is the country’s push to develop hydroelectric power plants throughout its northern and Amazon regions.

Work has already begun on the Belo Monte project in the state of Pará. When completed, Belo Monte will be the third largest hydroelectric project in the world, but damming the Xingu River is expected to displace more than 20,000 people, and flood 230 square miles of rainforest. Activists say the arrival of tens of thousands of workers also will contribute to deforestation.