Hundreds of thousands of people are marching worldwide to demand action to stop climate change, but one protest in Paris was marred by violence.
More than 2,000 events are happening globally on the eve of a UN summit in the French capital.
In Paris, police fired tear gas at a large group of demonstrators gathered in the Place de la Republique.
They were apparently protesting against France’s state of emergency, and have been disowned by the main organisers.
The order, banning public gatherings, was put into place after the 13 November attacks in the city, in which 130 people died.
Many of those involved in the clashes wore masks or covered their faces. Protesters and police walked over candles and tributes left for the attack victims at a makeshift memorial.
President Francois Hollande said it was “scandalous” that the clashes happened “where flowers and candles have been left in memory of those who were killed by the terrorists’ bullets”.
He added that the clashes had “nothing to do with those who protect the environment”.
Nicholas Haeringer, of the campaign group 350.org, said protests should still be allowed to continue, adding that campaigners would “stand against any attempts by the French authorities to use the incidents this afternoon to unnecessarily clamp down on civil liberties”.
Earlier in Paris, a human chain was formed by hundreds linking arms along the 3km (1.9 miles) route of a march that was called off after the 13 November attacks.
A gap in the chain was left in front of the Bataclan concert hall, where 89 people were killed.
Hundreds of pairs of shoes were left on Place de la Republique to remember those left frustrated in their plans to march.
Among them were a pair donated by Pope Francis, who has called for urgent action on climate change.
Elsewhere in Europe, organisers said some 10,000 people turned out in Berlin. with close to double that in Madrid.
An estimated 50,000 people took part in a march in central London, where opposition leader Jeremy Corbyn addressed crowds.
Activists want action to limit the rise in the average global temperature to 2C (3.6F) above pre-industrial levels.
In Sydney, many people carried placards reading “There is no Planet B” and “Solidarity on a global scale”.
“In 10 years’ time our children are going to say, ‘Mum, did you know about this? What was everyone doing’?” said Kate Charlesworth, a doctor and mother, in Sydney.
Mayor Clover Moore tweeted to say that, according to organisers, the true number of demonstrators was “at least 45,000″, which would make it the biggest ever such march in Sydney, if confirmed.
“Those who did the least to cause the problem are feeling the impacts first and hardest, like our sisters and brothers in the Pacific,” said Oxfam campaigner Judee Adams in a protest of some 5,000 people in Adelaide.
Further demonstrations are being held in Sao Paulo, Mexico City and New York, among other cities.
About 150 world leaders are due to attend the Paris talks including US President Barack Obama, China’s Xi Jinping, India’s Narendra Modi and Russia’s Vladimir Putin.
Mr Hollande says he hopes a deal can be achieved, but that it will not be easy.
“Man is the worst enemy of man,” he said. “We can see it with terrorism. But we can say the same when it comes to climate. Human beings are destroying nature, damaging the environment.”
UN climate conference 30 Nov – 11 Dec 2015
COP 21 – the 21st session of the Conference of the Parties – will see more than 190 nations gather in Paris to discuss a possible new global agreement on climate change, aimed at reducing greenhouse gas emissions to avoid the threat of dangerous warming due to human activities.
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