World leaders have opened the UN climate conference in Paris known as COP21, aiming to forge a deal to limit global warming to 2°C.
Both US President Barack Obama and French President Francois Hollande issued rallying cries to officials on Monday, urging them to commit to reducing emissions.
Obama said his country recognised its role in contributing to the climate change crisis, and would take the lead in fighting against it.
He added that the meeting symbolised a global "act of defiance" that proves the world stands undeterred by attacks linked to the Islamic State of Iraq and the Levant (ISIL) group in Europe and beyond.
"What greater rejection of those who would tear down our world than marshalling our best efforts to save it," Obama said.
"Our nations share a sense of urgency about this challenge and a growing realisation that it is within our power to do something about it."
"One of the enemies we'll be fighting at this conference is cynicism ... our progress should give us hope during these two weeks."
EARTHRISE: Long Road to Paris
Earlier, Hollande called on developed states to take ownership of the crisis and help countries threatened by changing weather patterns.
"My thoughts go out to those islands that may soon disappear ... developed countries must take responsibility. They are the ones who for years emitted the most greenhouse gases," Hollande said.
Pope Francis also added his voice to the mix, saying that it was "now or never" for leaders to tackle global warming.
"Every year the problems are getting worse. We are at the limits," he said. "If I may use a strong word I would say that we are at the limits of suicide."
Heads of 147 state and government are in the French capital for the start of two weeks of intense negotiations.
Monday's agenda included speeches by the leaders and environment ministers from a number of countries, including Germany, Iceland, Peru and Scotland.
|David Attenborough: I'm here to solve problems of distributing, gathering and storing power from non-polluting sources
A total of 25,000 official delegates hope to secure a legally binding accord for every country to cut carbon emissions.
The accord needs to come into effect from the year 2020, when current commitments from the Kyoto Protocol run out.
The talks hope to resolve the concerns of large developing countries, such as India, that say they are dependent on fossil fuels to spur economic growth, and richer countries, which are moving towards adopting cleaner energy sources but continue to be a big source of emissions.
Smaller and poorer countries that are more vulnerable to the effects of climate change are pressing for a deal that prioritises their safety.
Al Jazeera's Nick Clark, reporting from Paris, said China had previously been seen as an obstacle in reaching a deal, but its leaders had recently stressed the importance of finding renewable alternatives to fossil fuels.
"China has been a big spoiler in previous climate conferences but it seems to have changed its ways and pollution is one of the reasons why ... so they really want to clean up their act on that front," he said.
Obama is due to hold meetings with other world leaders during his two days in Paris, including with Chinese President Xi Jinping and Indian Prime Minister Narendra Modi.
The US leader added: "Let's secure an agreement that builds in ambition where progress paves the way for regularly updated targets. Targets that are not set for each of us but by each of us, taking into account the differences that each nation is facing.
"We know the truth - that many nations have contributed little to climate change but will be the first to feel its most destructive effects.
"I believe in the words of Martin Luther King Jr that there is such a thing as being too late. When it comes to climate change, that hour is almost upon us."
Inside Story: Can world leaders reach agreement on climate change?
The summit comes amid violent clashes between French police and anti-global warming activists who marched despite a ban on public demonstrations brought in after the Paris attacks.
Police used tear gas and pepper spray to disperse the protesters during one of several events urging world leaders to take stronger measures to prevent climate change on Sunday.
Earlier in the day, in an effort to circumvent security measures implemented after the November 13 attacks, thousands of shoes were placed in the city's sprawling central plaza to represent citizens urging a climate agreement.
Organisers of the display said that the Vatican donated a pair of shoes bearing the name of Pope Francis.
Hundreds of people also formed human chains in the late morning, interlinking arms and hands along the sidewalks between the central Place de la Republique and the Bataclan concert hall, where nearly 100 people were killed in one of the attacks.
|An artwork entitled One Heart One Tree by artist Naziha Mestaoui is displayed on the Eiffel tower [AP Photo/Thibault Camus]
Source: Al Jazeera And Agencies