At least 171 countries began signing the Paris climate agreement during a ceremony at United Nations headquarters on Friday, marking the largest number of nations ever signing an international agreement in a single day.
The agreement, whose text was passed by nearly 200 countries in Paris last December, is the first universal action plan intended to mitigate the impacts of climate change and hold the rise in global average temperature to below 2 degrees Celsius, compared to pre-industrial levels.
"This is history in the making," UN Secretary General Ban Ki-moon said. "We are breaking records in this chamber, and that is good news."
Ban noted that 15 countries, mostly small-island nations, would also deposit their instruments of ratification at the United Nations, certifying that they have put the Paris agreement into their national legislation.
"As you show by the very act of signing today, the power to build a better world is in your hands," Ban said.
French President Francois Hollande, whose country chaired and hosted the climate summit last year where the agreement was passed, called on countries, especially developed ones, not only to meet but to exceed the pledges they have made.
"The entire world must come together," he said. "Humanity is capable of the best to the extent that it is capable of the worst. Today, we have ensured that it is humanity's best that shines."
Hollande was the first leader to sign the agreement in the UN General Assembly hall.
Actor Leonardo DiCaprio, a UN messenger of peace focussing on climate change, called for global leadership to take unprecedented action to stop climate change, including abandoning the use of fossil fuels.
"Our planet cannot be saved unless we leave fossil fuels in the ground where they belong," DiCaprio said.
Chinese Vice-Premier Zhang Gaoli, whose country is one of the major emitters of greenhouse gases globally, said that China will ratify the Paris climate agreement before September when it hosts a summit for 20 of the largest countries.
US Secretary of State John Kerry hailed the Paris agreement as a definitive acknowledgement of the reality of climate change and said its power was in its ability to stir the world's economies to develop green technology and a new energy future.
Kerry, who signed the document with his granddaughter on his lap, said the US will also ratify the agreement before the end of the year.
The first opening statement of the ceremony was made by Getrude Clement, a 16-year-old radio reporter from Tanzania, reflecting the crucial importance of the agreement for future generations.
"We expect more than words on a paper and promises, we expect action, action on a big scale, and we expect action today, not tomorrow," Clement said.
"I expect to return to Tanzania and tell my friends that the future is ours and the future is bright."
The signing of the agreement on Friday, the first day it is open for signatures, marks a major step toward implementation.
The agreement will go into effect 30 days after 55 countries accounting for at least 55 per cent of global greenhouse emissions ratify it.
The number of countries signing the agreement will surpass the previous record of 119 for an international agreement on opening day.
Islamic leaders from around the world will also present the UN with a declaration in support of the climate agreement.