The two-week meeting of 2,000 delegates from 160 countries will seek to translate into a detailed "rule book" principles adopted in July in Bonn to cut significantly in the next decade emissions of so-called greenhouse gases blamed for pushing up global temperatures.
The treaty, known as the Kyoto Protocol and forged in 1997 in Japan, must be ratified by a majority of industrial nations responsible for global warming to take effect.
It aims to cut emissions of greenhouses gases by an average of 5 percent below 1990 levels by 2012.
The United States has already abandoned the Kyoto pact, with the Bush administration contending that U.S. industry would become uncompetitive if Washington ratified it. In the last conference - in Bonn, Germany - the United States stood alone while 178 other countries endorsed the final political agreement.
Moroccan police sealed off the Marrakech conference center and banned cars as a security measure. Outside, U.N. security officers screened the technical experts entering the meeting, which will conclude with a three-day gathering of Cabinet ministers or high-level policy makers.
Officials say the 2,000 U.S. citizens living in this North African country have faced no threats, other than two false anthrax alarms at the U.S. Embassy and the American school in the capital, Rabat. Twice, authorities have declined requests to stage demonstrations in Rabat against the U.S.-led bombing in Afghanistan.
"The fact that our conference has been held as planned with this important number of participants is a strong reply to those who believe that they can stop humanity on the path of progress," said the conference chairman, Moroccan Environment Minister Mohamed Elyazghi.
Still, several U.S. and Japanese non-government organizations canceled plans to send lobbyists to Marrakech.
"I'm not worried about traveling. I'm concerned about onsite security for such a high-profile meeting," said Glenn Kelly, head of the Global Climate Coalition which campaigns against the Kyoto pact, speaking from Washington.
Philip Clapp, of the pro-Kyoto National Environment Trust, said: "I can't justify sending staff to an Islamic country now."
"In Marrakesh, the focus will be on completing the translation of the Bonn agreements into legal language," Dutch Environment Minister and outgoing conference chairman Jan Pronk told the opening plenary session.
A political compromise on the main issues was reached in Bonn three months ago and the Marrakesh session is expected to produce a legally binding document.
"You can put the icing on the Bonn cake," Pronk told delegates, urging them to set aside political differences. "Don't renegotiate a political agreement already reached, just work it out," he said.
The Greenpeace environmental group sounded a less optimistic note, saying rules already agreed were so weak that they were unlikely to lead to a reduction of greenhouse emissions.
"Even the Protocol's nominal target of 5 percent reduction hardly started the process of making the 80 percent reductions needed to prevent dangerous levels of climate change," said Bill Hare, Greenpeace climate policy director.
The Marrakesh conference "must not be muddled with diplomatic doubletalk ...The processes that underpin the Protocol must be transparent and open for public participation," he added in a communique.
In the absence of the United States, the world's top industrial power and its biggest polluter, the support of the 15 European Union countries and Russia has become critical to guarantee success in Marrakesh, delegates said.
The treaty will enter into force if ratified by 55 countries responsible for 55 percent of emissions in 1990. Russia produced about 17 percent of those 11 years ago.
So far, 40 nations have ratified the treaty, including one industrialized country, Romania.
Michael Zummit Cutajar, executive secretary of the U.N. Climate Change Convention, said there no obstacles on the major issues.
From Russia "what's on the table is larger allowances for the use of sinks," he told a news conference.
"Sinks" is jargon for the forests and farms that absorb carbon from the earth's atmosphere. The deal allows Russia, but also Canada and Japan, to use these widely to reach their goals.
Moroccan Environment Minister Mohamed El Yazghi, who was elected conference chairman, said last months's suicide attacks on New York and Washington shocked everyone, but the U.N. climate talks were a clear answer to those who thought they could stop human progress.
"We don't have the right to fail," Yazghi told delegates, urging them to avoid raising new demands.
Acknowledging that the world's attention might be elsewhere, with the ongoing U.S.-led strikes on Afghanistan, he said climatic dangers were not the least of the dangers threatening mankind, which must act as one family.
The Marrakesh meeting, attended by some 4,000 people including non-governmental organizations and the press, is known as COP7, the seventh conference of the parties to a U.N. treaty signed in 1992 at the first Earth Summit in Rio de Janeiro.
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