"Mexico promises to reduce emissions of 50 million tonnes of carbon dioxide per year starting now," Calderon told foreign ambassadors, activists and scientists gathered in the steamy ecopark of Xcaret on the Yucatan Peninsula to mark World Environment Day.

Calderon, president of one of the world's most biodiverse countries, also presented plans to roll out energy-saving lightbulbs and incorporate environmental education in schools, as well as announcing five new protected areas, including a whale shark reserve off Mexico's Caribbean coast.

Activists from Greenpeace -- who failed to get closer to events for the Mexico-hosted day than a motorboat trailing a banner off the coast -- praised the president's words but said the green plans did not fit within the country's energy policy.

"The worst thing is that it's all incongruent with the energy policies," Raul Estrada from Greenpeace said from outside the event by telephone.

Estrada cited a recent report from the federal electricity commission on plans to build new carbon plants, which named fuel burning as the country's second technology for producing electricity, after oil.

Stemming carbon dioxide emissions from burning fossil fuels is central to tackling climate challenge.

Mexico should invest more in alternative energies, Estrada said, and criticized the government's choice of the heavily-developed Yucatan for hosting the environmental event.

"There's a schizophrenia in these policies," Estrada said.

The annual UN environment day this year focused on climate change, as worldwide negotiations increased on new rules for slashing emissions of greenhouse gases, ahead of a key summit in Copenhagen in December.

"Climate change is happening, it is happening ever faster and we are too slow in responding," said Achim Steiner, executive director of the United Nations Environment Program.

Mexican Finance Minister Agustin Carstens meanwhile presented a project to evaluate the cost of climate change in Mexico, inspired by the 2006 Stern Review, which become the benchmark for calculating the economic cost of tackling climate change.

The UN-sponsored World Environment Day began 37 years ago and takes place annually on June 5.

Ahead of the event, Mexican billionaire Carlos Slim, one of the world's richest men, promised to invest 50 million dollars in a joint investment project with the Mexican government and the World Wildlife Fund, in a move lauded by experts as a model for emerging countries to follow.

"Big investments which are not apparently profitable in the short term are necessary in order to stabilize the emission of greenhouse gases," Slim said here Friday.

"Without a doubt these cost a lot less than if climate change continues its trend."