A total of 175 countries have signed the Paris climate agreement at the United Nations in New York City, a record for a one-day signing of an international accord, the UN says.
French President Francois Hollande and Canada's Justin Trudeau joined US Secretary of State John Kerry for the record turnout that has boosted hopes of quick action on combating global warming.
"This is a moment in history," UN Secretary-General Ban Ki-moon said.
"Today you are signing a new covenant with the future."
Held on Earth Day, the ceremony comes four months after the hard-won deal was clinched in Paris and marks the first step toward binding countries to the promises they made to cut greenhouse gas emissions.
While the United States, China and India — the world's top greenhouse gas emitters — were not represented by their highest officials, some 60 heads of state and government were set to be among the signatories.
The Paris Agreement will come into force as soon as 55 countries responsible for 55 per cent of the world's greenhouse gases have ratified the accord.
The target date for the agreement to begin is 2020, but momentum is building to ensure the accord enters into force much earlier.
Mr Hollande called on governments to quickly ratify the Paris deal and singled out the European Union, saying the 28-nation bloc should "lead by example" and give final approval before the end of the year.
China and the United States have said they will ratify this year and are pushing for others to follow suit so that the agreement becomes operational possibly as early as late 2016 or 2017.
But the European Union may take up to a year and half for the ratification.
'The world is now watching': Leonardo DiCaprio
Caught in election-year turmoil, the United States plans to ratify the Paris accord with an executive agreement, bypassing the Senate and setting up a complex process for any future president wishing to pull out.
Moving quickly, 15 countries, mostly island states, have already fully approved the agreement and will formally present the completed ratification to the United Nations.
Oscar winner and environmental campaigner Leonardo DiCaprio also addressed the gathering.
"The world is now watching," he said.
"You will either be lauded by future generations or vilified by them.
"We can congratulate each other today, but it will mean absolutely nothing if you return to your countries and fail to push beyond the promises of this historic agreement."
Last month was the hottest March in modern historyand 2016 is shaping up as a record-breaking year for rising global temperatures.
This year's El Nino — dubbed "Darth Nino" — is wreaking havoc, with droughts, floods, severe storms and other extreme weather patterns.
"We are in a race against time," Mr Ban told the gathering at the UN General Assembly.
Agreed by 195 nations, the Paris deal sets the goal of limiting global warming to two degrees Celsius above pre-industrial levels by moving to clean energy.
Mr Ban stressed that the window for keeping the global temperature rise below two degrees Celsius was rapidly closing.
The Paris deal also places an onus on rich nations to help poorer ones make the transition to a low-carbon economy.
The agreement was seen as a triumph for Mr Ban who pushed for the deal throughout his tenure and has listed the agreement in Paris as one of his proudest moments as UN chief.