From Asia to the Americas via Europe and the Middle East, activists around the planet have protested in an effort to mobilize public opinion against global warming 50 days ahead of a crucial UN climate summit.
Many of the thousands that gathered on the steps of Sydney's iconic Opera House to kick off the event waved placards bearing the logo 350, a figure scientists believe is the maximum parts per million of CO2 that the atmosphere can bear to avoid runaway global warming.
In New York's Times Square, a crowd of demonstrators gathered as giant screens beamed in images from around the world. Organizers told the activists that events had taken place in "more than 180 countries" at 5,200 events.
In France, politicians received a "wake up" call from several hundred Parisians who chose clocks as their symbol.
Protesters who met in a central square in Paris had set their alarm clocks and mobile phones to ring at 12:18 pm (1018 GMT) in reference to the closing date of the UN summit in Copenhagen, which lasts from December 7-18.
The summit is considered crucial as world leaders will try to thrash out a new treaty to curb greenhouse gas emissions in place of the Kyoto Protocol, which expires in 2012.
But senior officials from the United States and China, the world's two largest polluters, have warned the talks may fail.
There is growing concern that a treaty deal in Copenhagen could be hampered by issues that include US domestic politics and the problems of securing agreement between developed and developing countries.
In Berlin, some 350 protesters wearing masks with the face of German Chancellor Angela Merkel came together in front of the Brandenburg Gate in the city center.
In London, more than 600 people gathered beneath the London Eye Ferris wheel by the River Thames to arrange themselves into the shape of the number five, according to organisers Campaign against Climate Change.
An aerial photograph of the event will be added to pictures of a giant "three" and "zero" from around the world.
"Hundreds of thousands of people are taking part (globally) and for us that's so important, to have people out on the streets," campaign activist Abi Edgar told AFP. "We want serious action on climate change and we want it now."
Across the Thames, some 100 musicians playing trumpets, trombones, saxophones and clarinets gathered outside parliament to play the same note -- an F, made by the frequency of 350 Hz -- for 350 seconds, organizers said.
In the Lebanese capital Beirut hundreds of activists, many wearing snorkels, held demonstrations in key archaeological sites.
They gathered around the Roman ruins in central Beirut, in the ancient eastern city of Baalbek and along the coast, carrying placards bearing the 350 logo.
"It's not the first time Beirut will have gone under water," Wael Hmaidan of the IndyACT group organizing Beirut's protests told AFP, explaining the goggle-wearing, "but this time it's going down because of climate change, and not earthquakes."
Environmental activists in the Turkish metropolis of Istanbul staged their protest in a boat, unfurling a banner reading "Sun, wind, right now!" under the main bridge linking Asia and Europe over the Bosphorus Strait, Anatolia news agency reported.
They then sailed to the ancient Maiden's Tower, which sits on a tiny islet in the Bosphorus, and unfurled another banner reading "Jobs, climate, justice," the report said.
Events in Asia included demonstrators in Dhaka riding bicycles to highlight one way of cutting emissions.
In Jakarta, around 100 students from the London School of Public Relations gathered to form the symbolic number 350, coordinator Candy Tolosa said on Detik.com news website.
The following YouTube video animation was released via 350.org, and it explains the reasoning behind today's climate protests, and the significance of the number 350:
With AFP reporting...