The Kyoto accord, which aims to curb the air pollution blamed for global warming, has come into force seven years after it was agreed.
Industrial pollutants are blamed for global warming
The accord requires countries to cut emissions of carbon dioxide and other greenhouse gases.
Some 141 countries, accounting for 55% of greenhouse gas emissions, have ratified the treaty, which pledges to cut these emissions by 5.2% by 2012.
But the world's top polluter - the US - has not signed up to the treaty.
The US says the changes would be too costly to introduce and that the agreement is flawed.
Large developing countries including India, China and Brazil are not required to meet specific targets for now.
'Out of control'
The ancient Japanese capital of Kyoto, where the pact was negotiated, is hosting the main ceremony marking the treaty's coming into force.
Russia ratified the treaty in November 2004 - the crucial moment making the treaty legally binding.
Russia's entry was vital, because the protocol had to be ratified by nations accounting for at least 55% of greenhouse gas emissions to become valid.
This target was only met after Russia joined.
But the head of the UN Environment Programme, Klaus Toepfer, said Kyoto was only a first step and much hard work needed to be done to fight global warming.
"Climate change is the spectre at the feast, capable of undermining our attempts to deliver a healthier, fairer and more resilient world," he said.
Recent projections on planet warming made terrifying reading, he said, painting a vision of a planet that is "spinning out of control."
He said it would be Africa which bore the burden of the world's failure to act.
The protocol, which became legally binding at midnight New York time (0500 GMT) on 16 February, demands a 5.2% cut in greenhouse gas emissions from the industrialised world as a whole, by 2012.
Not just hot air - more than 140 countries have signed up
Each country has been set its own individual targets according to its pollution levels.
Growing developing countries China and India are outside the framework, a fact pointed out by US President George W Bush when he abandoned Kyoto as one of his first acts when taking office in 2001.
Japan's Prime Minister Junichiro Koizumi issued a statement welcoming the treaty but also calling on non-signatories to rethink.
"From now, we have to build a system in which more nations will work together under the common framework to stop global warming," he said.
Environmentalists held protests around the world to mark the treaty coming into force - with many targeting the US.
Speakers at the official ceremony include Nobel Peace prize winner Wangari Maathai.
Ms Maathai, an ecologist and Kenya's deputy environment minister, said the Kyoto Protocol would require not just efforts from governments and businesses, but also a change in the way people lived.
But even for countries that have signed up to Kyoto, meeting the goals could be difficult.
Canada, one of the treaty's first signatories, has no clear plan for reaching its target emission cuts. Far from cutting back, its emissions have increased by 20% since 1990.
And Japan is also unsure it will be able to meet its legal requirement to slash emissions by 6% from 1990 levels by 2012.
"Japan will make all efforts to respect the rules of the Protocol," said Takashi Omura, of the Japanese environment ministry. "It will neither be easy nor insurmountable."