Brazil is mounting a massive security operation for the UN Rio+20 conference on sustainable development, with 20,000 troops and police on alert to thwart any cyber and terrorist attacks.

While a navy frigate patrols Rio's Guanabara Bay, army troops have been tasked with protecting the Rio Centro convention center, the venue for the UN-sponsored event that will draw 116 world leaders on June 20-22.

The troops are also guarding the 38 hotels that house the official delegations and security is being tightened at major highways, airports, ports and strategic installations such as gas and water plants.

The UN conference, which marks the 20th anniversary of the Earth Summit -- a landmark 1992 gathering that opened the debate on the future of the planet and its resources -- is the largest ever organized, with 50,000 delegates.

The security measures also apply to the parallel "people's summit" which opens Friday at the Flamengo park in southern Rio.

That week-long event, called by civil society to denounce the "green economy" concept advocated by the official Rio+20 summit participants, is expected to draw nearly 20,000 people a day.

"We did very detailed planning, with a lot of people and equipment to ensure that the conference takes place in peaceful conditions," Defense Minister Celso Amorim said late last month.

"In today's world, cyber attacks are a dangerous threat. We have to be on our guard to guarantee the security of all the conference documents," Amorim then said.

But General Adriano Pereira Junior, the security coordinator for the conference, said there were no specific threats.

"Rio de Janeiro is a city which lives in peace. We do not view the communities as a risk factor and there is no plan to use armor to protect the city," he noted.

Unlike during the Earth Summit here 20 years ago, when Rio was one of Brazil's most dangerous cities, tanks will not be used to deter violence in the favelas (slums) but only to support the troops' logistics and movements, officials said.

Since 2008, security forces have moved in and pacified major favelas overlooking the city after wresting control from narcotraffickers who used to terrorize residents.

Specially trained troops are on standby to react to any terrorist attacks or against any chemical or bacteriological treats, according to the defense ministry.

And authorities said a 40-member task force has been set up to protect the telecommunications network from possible hacking threats.

Measures have also been taken to avert any traffic paralysis.

Rio+20, the fourth major summit on sustainable development since 1972, will debate prospects for a "green" economy that can balance economic growth with poverty eradication and protection of the environment.

Kicking off the summit Wednesday, Brazilian President Dilma Rousseff called on "all countries of the world to commit" to reaching an accord that addresses the most pressing environmental and social woes.

But it opens against a backdrop of discord and economic gloom, and there are fears over the draft summit communique after three rounds of preliminary negotiations left more than 75 percent of the paragraphs still to be agreed.