World leaders at a UN summit billed as the largest-ever gathering on climate change faced calls Tuesday to take bold action to reverse global warming.

"Today, we must set the world on a new course," United Nations Secretary General Ban Ki-moon said at the opening of the summit at UN headquarters in New York. "I am asking you to lead."

The meeting is the first of its kind since the Copenhagen summit on climate change ended in disarray in 2009 and is seen as crucial to build momentum ahead of the Paris conference in late 2015.

Diplomats and climate activists hope the summit attended by 120 leaders will pave the way for a deal to be reached in Paris on reducing greenhouse gas emissions after 2020.

But no-shows from the leaders of China, the world's biggest polluter, and India, the number-three carbon emitter, cast a cloud over the event.

France pledged up to $1 billion to the UN Green Climate Fund, which helps finance climate change reform in poorer countries, and the United States was expected to make pledges later in the day.

French President Francois Hollande said the Paris conference should deliver a "global and ambitious" deal and warned that climate change posed a "threat to world peace and security."

In her address, Brazil's President Dilma Rousseff said tackling climate change and development were not contradictory goals, saying "we have reduced poverty and we have protected the environment."

- DiCaprio at the podium -

Leonardo DiCaprio brought star power to the UN summit, urging leaders to stop treating global warming as if it were a fiction.

"As an actor, I pretend for a living. I play fictitious characters often solving fictitious problems," the actor told the summit.

"I believe mankind has looked at climate change in that same way -- as if it were fiction."

Sporting a ponytail and suit and tie, the star of "The Wolf of Wall Street" and "Titanic" said: "Now it is your turn, the time to answer humankind's greatest challenge is now. We beg of you to face it with courage and honesty."

The summit was being held after marches drew hundreds of thousands of demonstrators to the streets of cities worldwide on Sunday in a show of "people power" directed at leaders reluctant to tackle global warming.

Key players from the private sector also stepped into the fray to trumpet their commitment to greening, with Apple CEO Tim Cook announcing on Monday that the tech giant would prioritize low-carbon growth.

The Global Environment Facility, a broad partnership of governments, civil society and the private sector, pledged $3 billion in climate finance for the next four years, focused on cities and commodities suppliers.

The summit talks are separate from the negotiations held under the UN Framework Convention on Climate Change (UNFCCC), which will culminate with the Paris conference in December 2015.

The United Nations is seeking to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels, but scientists say current emission trends could hike temperatures to more than twice that level by century's end.

One recent report warned that a surge in carbon dioxide levels had pushed greenhouse gases to record highs in the atmosphere, increasing at their fastest rate in 30 years in 2013.

The 1997 Kyoto Protocol, the first international agreement to reduce emissions, has commitment periods, the last of which expired in 2012. But it has since been renewed. However the protocol was never ratified by the US.

Attempts to negotiate a new treaty ended in fiasco at the Copenhagen conference in 2009 and the pressure is on to avoid a repeat of that failure at the UN talks in Paris next year.