Activists mobilized in cities across the globe today for marches against climate change, with one of the biggest planned for New York, where celebrities, political leaders and tens of thousands of people were expected.

Hollywood actor Leonardo DiCaprio, former US vice president turned advocate Al Gore, UN Secretary General Ban Ki-moon and New York Mayor Bill de Blasio were all due to take part in what organizers hope will be the largest climate change protest in history.

The "People's Climate March" in New York has been endorsed by more than 1,400 organizations, including environment, faith and justice groups, as well as labor unions. Students have mobilized marchers from more than 300 college campuses.

The rallies, including another 2,500 around the world, take place ahead of a climate change summit hosted by Ban Tuesday on the sidelines of the United Nations General Assembly.

In the French capital, nearly 5,000 people protested, according to police estimates, many on bikes, with banners that read "Climate in danger" or "World leaders, act!"

"Before we could say we didn't know. Now we know. Climate change is already underway," Nicolas Hulot, the president's special envoy for the protection of the planet, told the crowd in central Paris.

In the southwestern city of Bordeaux, up to 700 people also took part in a climate change march.

Protesters held banners, one reading, "If climate was a bank, they would already have saved it."

Hundreds more protested in several other cities in France.

In Madrid, hundreds gathered in front of the Environment Ministry, brandishing signs with slogans including "There's no Planet B," "Change your life, not your climate," and "Our climate, your decision."

In Cairns, Australia, where finance ministers from the G20 nations were meeting, more than 100 people wearing green paper hearts around their necks gathered outside the venue.

They repeatedly chanted "Every dollar spent, every single cent, 100 percent, green energy" and carried banners including one that read: "Add climate change to the G20."

"I'm here because I'm a parent, I'm here because I'm a scientist, I understand what climate change means for our planet, our children, our economy, our health," said John Rainbird, an Australian biologist.

Hundreds also gathered in Sydney, Australia, and in New Delhi, India, where around 300 protesters carried placards that read "I want to save forests" and "Coal kills", as they shouted slogans and danced to pounding drum beats.