GENEVA, OCT. 29 -- The top U.N. environment official today urged immediate steps by industrialized countries to curb global warming, saying delay will only worsen a disaster looming over future generations.

Mostafa K. Tolba, head of the U.N. Environment Program, said a "complete change in attitudes and lifestyles" is needed to avert a danger "potentially more catastrophic than any other threat in human history."

A decade of research has made clear that only drastic reductions in emissions of carbon dioxide and other heat-trapping "greenhouse gases" will stop warming of the planet, Tolba told the opening session of a U.N. climate conference.

He said "large gaps in our knowledge" about the phenomenon should not be an excuse for inaction.

About 500 scientists attending the 10-day conference are expected to draft a new program proposing ways to fight the greenhouse effect.

A final meeting of about 80 ministers is expected to draw several heads of government, including Margaret Thatcher of Britain and France's Michel Rocard. The United States, still opposed to binding emission cuts, has sent a lower-level delegation.

The conference is supposed to launch formal negotiations on an international convention against global warming, to be ready for signing by mid-1992. Ministers are expected to provide the basis for the talks in a final declaration Nov. 7.

But Tolba said industrialized nations, which contribute three-quarters of all man-made greenhouse gases, must cut emissions even before a convention is in place. "The longer we delay, the worse the disaster will be for our children, and for our children's children," he said.

He said curbing emissions will be costly and requires great changes in "economic, legal and institutional systems." Developing countries would need more time and Western help.

"We are used to war and its price. We must now learn to live in peace, and to know and pay its price," said Federico Mayor, director-general of the U.N. Educational Scientific and Cultural Organization (UNESCO).