Tens of thousands rallied in Melbourne on Friday to launch a global campaign for climate action ahead of a key summit in Paris, where marches were cancelled amid security fears.

Protesters young and old echoed the call for a phase-out of fossil fuel blamed for climate-altering greenhouse gas emissions, waving placards reading: "There is no Planet B" and "Stop messing with my future".

Police said about 40,000 people joined the Australian rally -- the first of more than 2,300 events planned in about 150 countries for this weekend.

Protest organisers have set up a website, www.march4me.org, where people in other cities can sign up to symbolically march for those who cannot do so in Paris.

Under heightened security two weeks after France's worst terror attack, some 150 heads of state and government will on Monday launch a highly-anticipated UN conference tasked with inking a 195-nation climate rescue pact.
The negotiations themselves, conducted by rank-and-file bureaucrats, have been brought forward to Sunday, the same day tens of thousands of people will take part in Global Climate Marches to beat the drum for urgent action.

Demonstrations are planned for Saturday and Sunday in cities as diverse as London, New Delhi, Kampala, Manila, Kyoto, Austin and New York, Cairo and Sao Paulo.

But in Paris itself, French authorities cancelled two rallies following the coordinated onslaught by gunmen and suicide bombers which killed 130 people out for dinner, drinks and a concert on November 13.

The city's main climate rally was to have started on Sunday from a square very near the scene of some of the violence, followed by another on December 12 at Le Bourget airport, where the conference is taking place.
"It is... important for people everywhere to march on the weekend of November 29 on behalf of those who can't, and show that we are more determined than ever to meet the challenges facing humanity with hope, not fear," Emma Ruby-Sachs of activist umbrella group Avaaz said Friday.

In the French capital, still under a state of emergency, activists now plan to create a two-kilometre (1.2-mile) human chain along the original march route on Sunday.

They will break the chain as they pass the Bataclan concert hall, where the worst violence claimed 90 lives, as a mark of respect to the victims.

- History in the making -

Activists also plan to leave scores of shoes on Place de la Republique square to symbolise the thousands left frustrated in their plans to march.

On Monday, Le Bourget will host the largest climate gathering of heads and state and government ever.
Leaders including Barack Obama, Xi Jinping, Narendra Modi and Vladimir Putin are meant to inject a sense of mission into the conference tasked with crafting the first truly universal climate pact.

The goal is to limit average global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels by curbing fossil fuel emissions blamed for climate change.

This week, the UN's weather body said the average global temperature for the year 2015 is set to touch the halfway mark at 1 C.

And analysts say voluntary carbon-curbing pledges submitted by nations to bolster the Paris pact, even if fully adhered to, put Earth on track for warming of 3 C.

A report by the Asian Development Bank Friday warned that climate disasters may double in the next two decades unless CO2 emissions are cut.

The Paris conference will gather some 40,000 people, including 10,000 delegates from 195 countries, plus journalists, observers, scientists, exhibitors and visitors.

About 2,800 police and soldiers will secure the conference site, and 6,300 others will deploy in Paris.

Obama has urged world leaders not to let a recent string of extremist attacks around the world deter them from coming to Paris, where questions of world security are likely to intermingle with climate on the conference agenda.

"It's absolutely vital for every country, every leader, to send a signal that the viciousness of a handful of killers does not stop the world from doing vital business," the US President said last Sunday.

Climate will also feature strongly at a Commonwealth summit in Malta on Friday.