London's mayor Ken Livingstone has teamed up with Bill Clinton in a bid to help major cities around the world cut greenhouse gas emissions.
Prime Minister Tony Blair was also at the event
London, with 21 other cities, has signed up to the former US president's new Clinton Climate Initiative (CCI).
The event in Los Angeles announced the partnership of the CCI with the Large Cities Climate Leadership Group, which is chaired by Mr Livingstone.
Urban areas are responsible for over 75% of all greenhouse gas emissions.
The initiative hopes to create a purchasing consortium that will pool the purchasing power of the cities to lower the prices of energy saving products and speed up the introduction of green policies.
It also wants to create an internet-based communication system to share best practices and to measure the effectiveness of each city's efforts.
The CCI is part of the Clinton Foundation, which has used similar methods to lower AIDS drug prices for members of its purchasing consortium.
"The fate of the planet that our children and grandchildren will inherit is in our hands, and it is our responsibility to do something about this crisis," said Mr Clinton.
"The partnership between my foundation and the Large Cities Climate Leadership Group will take practical and, most importantly, measurable steps toward helping to slow down global warming, and by taking this approach I think we can make a big difference."
Mr Livingstone said: "There is no bigger task for humanity than to avert catastrophic climate change. The world's largest cities can have a major impact on this.
"Former President Clinton and his Foundation have proved that they can intervene decisively to make a real impact on one of the world's biggest problems, AIDS.
"Our aim is simple - to change the world."
Mr Clinton and Mr Livingstone were joined by British Prime Minister Tony Blair, Los Angeles Mayor Antonio Villaraigosa and San Francisco Mayor Gavin Newsom.