The "Terminator" action hero was the star attraction at the opening of the United Nations World Environment Day conference in San Francisco, where mayors from the world's most populated cities are gathering to draw up plans to protect increasingly-fragile urban environments.

The brawny icon signed an executive order mandating cuts in greenhouse gas emission in the most populous US state, even though Washington has refused to ratify the Kyoto Protocol on the environment.

The decree provides that greenhouse gas emissions in California be cut to year 2000 levels in the next five years, and that they be 80 percent below 1990 levels by 2050.

"California will continue to be a leader in the fight against global warming and protecting our environment," said the movie tough guy, who stole the show and received a standing ovation.

"By working together we can meet the needs of both our economy and environment," he added.

The move to slash greenhouse gas emissions in environmentally-conscious California comes after his fellow Republican, President George W. Bush, refused to put the Kyoto Protocol to Congress for ratification.

"We have no choice but to meet this challenge," Schwarzenegger said as he signed his executive order.

"Now, we know better. If we don't do anything about it, it will be an injustice. I challenge everyone to match our commitment."

Mayors from 66 cities ranging from Istanbul, to London, Shanghai, Zurich, Calcutta and Panama City are taking part in the "Green Cities" accord session, the first time the annual meeting has been held in the United States.

San Francisco is rolling out the green carpet for this years's UN Environment Day conference to mark the 60th anniversary of the signing of the world body's charter here in June 1945.

The mayors from across the globe took on the roles of frontline generals in the battle to protect the environment.

For the first time in human history, more than half the world's population lives in cities, San Francisco Mayor Gavin Newsom observed while welcoming his peers from around the globe.

More than 300 events ranging from recycling plant tours, to "eco-workshops" -- on topics including smart growth and sustainable energy -- to a Golden Gate Bridge walk and a fashion show featuring environmentally-friendly clothing are scheduled.

The mayors will see the latest in green technologies, hear about what is working and what isn't and can then take ideas home, said Earth Council Alliance Chairman Tommy Short.

"The mayors are in the trenches, literally on the street facing problems. They have to deal with it every day and their decisions affect millions of people," he said.

The alliance council, founded to avoid the lack of follow-through that undermined the 1992 environmental summit in Rio de Janeiro, does more than half its work in China.

"We see China as having the biggest need in the next 10 to 15 years because of its tremendous growth," he said during a mayors gathering at the Natural World Museum here.

"They are at an early stage of development and trying to get it right the first time around," Short said at the museum that displays items including hybrid and hydrogen-fueled vehicles and a "scrap house" built of discarded material including telephone directories.

Schwarzenegger warned the mayors it was time to take serious action to protect the environment.

"We know the signs. We see the threats. We know the time for action is now," the celebrity governor said. "We must leave a better world for our children and for their children."

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