French President Francois Hollande has called on nearly 200 nations to adopt "the first universal agreement on climate".
He told delegates at the Paris conference that the deal would be "unprecedented" in the history of international climate talks.
UN secretary-general Ban Ki-moon also made an impassioned plea to world diplomats negotiating the deal.
"The whole world is watching. Billions of people are relying on your wisdom," he said.
"The time has come to acknowledge that national interests are best served by acting in the international interest. We have to do as science dictates. We must protect the planet that sustains us. We need all our hands on deck."
French foreign minister Laurent Fabius said a "final" draft of the agreement, which would be legally binding, is being translated and distributed to nations.
He said the accord would aim to keep the rise in global temperatures "well below" 2C from pre-industrial times to the end of this century and " endeavour to limit" them to 1.5C - a key demand of small island nations and other poor and vulnerable countries.
US Secretary of State John Kerry said he believes the draft pact is "teed up" to be adopted on Saturday.
"It should be good but we'll see. Little things can happen but we think it's teed up," Mr Kerry said.
Activists gathered near the Eiffel Tower are already denouncing the potential accord as insufficient to protect the planet.
As the Paris meeting continued, protesters from environmental and human rights groups gathered in the city to call attention to populations threatened by rising seas and increasing droughts and floods.
Thomas Coutrot, of advocacy group Attac, said the accord is an optical illusion that "masks" a lack of serious policy changes like abandoning oil altogether.
"This accord is unacceptable," he said. He called for declaring "a state of climate emergency".
Guillaume Durin, of Alternatiba, said: "We are convinced the agreement won't be enough."