More than 2,000 climate events are planned in cities including Sydney, Jakarta, Berlin, London, Sao Paulo and New York, making it one of the biggest days of action on climate change in history, organizers say.
Activists in France scaled back their plans when the government imposed a state of emergency after the attacks two weeks ago killed 130 people, banning the planned demonstration in Paris, meant as the biggest of all.
In France, activists plan to form a static human chain, formed by about 3,400 people joining arms along what had been the original 3 km (1.9 miles) route through central Paris from the Place de la Republique to Place de la Nation.
"This is a moment for the whole world to join hands," said Iain Keith, campaign director for Avaaz, one of the organizers.
Separately, more than 10,000 demonstrators who had planned to come to Paris have instead sent shoes to form a big pile in a sign of solidarity. Organizers said the Vatican even sent a pair to represent Pope Francis.
Alix Mazounie of French Climate Action Network said the activists reckoned a human chain would not violate the state of emergency.
"This is not civil disobedience," she said. The chain would break, for instance, wherever it crossed a road to avoid disrupting traffic.
But, underscoring security worries, France put 24 green activists under house arrest before the summit, Interior Minister Bernard Cazeneuve said on November 28, saying they were suspected of planning violent protests at the talks.
Still, Foreign Minister Laurent Fabius welcomed the worldwide demonstrations, which organizers say will include concerts, rallies, bicycle rides and a march by 1,000 Maasai in Tanzania urging more renewable energy.
"It is very positive," Fabius said, for governments to feel public pressure to act. Many environmental activists want a phase-out of fossil fuels and a shift to 100 percent renewable energies by 2050.