Mayors from some of the world's largest urban areas called on cities to unite and take the lead in tackling climate change Tuesday, at a summit in New York devoted to protecting the environment.

"As cities produce three-fourths of the carbon emissions, we must act," said London Mayor Ken Livingstone, the head of the C40 large cities, describing climate change as "the single biggest threat to the future of humanity." "Whatever the discussions within our governments, as cities we are not waiting," he told leaders from 46 of the world's most polluted cities, from Cairo to Shanghai and Los Angeles to Bangkok.

Livingstone said the summit aimed to "create a critical mass that puts the world on the path to avoid a catastrophic climate change... We came to take decisive actions to reduce our own carbon emissions," he said. The summit, which opened late Monday and runs through Thursday, is expected to include several joint initiatives that harness the cities' combined purchasing power.

The event is being organized in conjunction with the Clinton Climate Initiative, part of the foundation set up by former US president Bill Clinton, who is due to address the summit on Wednesday. The summit has also attracted dozens of major corporations, including GE, Deutsche Bank, Swiss Re, JP Morgan Chase, Shell and Siemens, who are either offering technological expertise or financial backing for green projects.

New York Mayor Michael Bloomberg criticized governmental inaction on climate change, telling delegates: "We need no new technology, we need no new invention, all that is required is political will." "If they don't act, we will. Shame on them but we cannot sit around and watch our environment deteriorate and put this world in jeopardy," he said. "We are willing to stand up, we think it is one of the seminal issues of our time."

Other topics up for discussion at the summit include beating traffic congestion, making water systems more efficient, adopting renewable energy sources, increasing recycling, reducing waste and improving mass transit. When London hosted the inaugural large cities summit in 2005, only 18 cities took part. With climate change now one of the most pressing hot-button issues, the number of cities represented has more than doubled.

"Even here in the United States things are beginning to move," said New York's deputy mayor, Daniel Doctoroff. "The time for debate is over, the time for action is now." He explained how a plan to manage an expected boom in the city's population over the coming decades had evolved into proposals to trim carbon emissions by 30 percent before 2030 and restrict vehicle access into Manhattan. "So many companies are now taking it seriously," he said.

Some 500 US mayors were also at the summit to show their objections to the policies of President George W. Bush, who has refused to sign up to the Kyodo Protocol, which commits countries to reduce greenhouse gas emissions. "Mayors took action because we have to, because the federal government was silent," said Douglas Palmer, head of the United States conference of Mayors.