Mayor Bloomberg and Mayor Livingstone of London may be rivals when it comes to luring financial service firms to their cities, but the two stood side by side yesterday in calling for action on climate change.

The mayors were at the C40 Large Cities Climate Summit — an event Mr. Bloomberg and President Clinton are hosting in Manhattan this week — and said there is no time to wait for federal governments to act.

“If they don’t act, we will,” Mr. Bloomberg said. “We cannot sit around and watch our environment deteriorate and put this world in jeopardy. The public wants action, and if you have a void, the mayors are going to fill that void.”

Mr. Livingstone praised his New York counterpart, terming “courageous” Mr. Bloomberg’s recent proposal to tax vehicles for traveling in Manhattan during peak hours. The London politician has already implemented such a charge in London, and is now looking to take it a step further by increasing fees for high-polluting cars.

“Can I just give you one word of warning?” Mr. Livingstone said during a news conference in Central Park. “There may be one or two people who will predict doom and gloom. Ignore them.”

Mr. Livingstone called climate change the “biggest problem facing humanity” and said the only thing required to tackle it is “political will.” He said his poll numbers shot up right after the London charge took effect.

The mayor of New Orleans, Ray Nagin, who was also on hand, said the cities have to take the lead. He said that when the federal government comes around to what the cities are doing, “everybody will be holding hands and singing ‘Kum Ba Ya.'”

The summit — a gathering of CEOs and representatives from cities around the world — was an international showcase for Mr. Bloomberg and his new environmental agenda. His keynote speech was translated into a slew of languages so that guests could listen on headsets.

At the news conference after the speech in Central Park, a reporter asked why, if the mayor is trying to conserve energy, the streetlights are turned on in the middle of the afternoon in the park.

A flustered but quick-witted Mr. Bloomberg said, “I have absolutely no idea why the streetlights in Central Park are switched on. I’m glad you brought that to the mayor’s attention. They shouldn’t be turned on. I assume they are controlled by some kind of an automatic device which is malfunctioning.”

After a quick side conversation with his press secretary, he added that the lights were needed to power the sound system. “If it’s not true, it just shows the creativity of my press secretary,” he joked.

Mr. Clinton and the mayor today will announce new “global climate initiatives” during a joint appearance at the summit.