Hundreds of thousands of people rallied around the world Sunday on the eve of a Paris summit aimed at averting catastrophic climate change, but violent clashes in the French capital soured the show of people power.
The global protests, including an emotional linking of hands near the heart of this month's terror attacks in Paris, were aimed at building grassroots pressure for an historic deal at the UN talks to limit global warming.
As US President Brack Obama and other world leaders began flying into Paris for Monday's official opening, negotiators vowed at a preliminary session to honour the victims of the attacks by forging an ambitious deal.
"The best way to honour the memory of those who have fallen, those who are victims of barbaric attacks, is to carry out what we have committed to," the co-chair of the talks, Ahmed Djoghlaf, told participants at a vast conference centre in Le Bourget, on the northern outskirts of Paris.
Deep emotions and tensions from the November 13 attacks, in which Islamist militants killed 130 people in a series of gun and suicide bomb assaults, were evident across the City of Lights on Sunday.
French authorities cancelled two climate demonstrations because of security fears.
But in a show of defiance against the militants and determination to have their voices heard on climate change, thousands of people in Paris gathered to hold hands and link up in a two-kilometre (1.2-mile) human chain.
"Hear our voices! We are here!" they chanted.
Protesters left a 100-metre (300-foot) gap in the human chain outside the Bataclan concert hall, the site where gunmen killed 90 people, as a mark of respect to the victims.
Instead of marching, activists placed thousands of pairs of shoes -- weighing more than four tonnes according to organisers -- on Place de la Republique square.
UN Secretary General Ban Ki-Moon left a pair of running shoes, and Pope Francis sent shoes to be placed on his behalf.
But a band of anti-capitalist militants infiltrated the protests, leading to clashes with riot police in the late afternoon and the detention of more than 200 people.
Police fired teargas at protesters, who pelted them with bottles and candles in Place de la Republique and chanted: "State of emergency, police state", referring to the post-attack protest restrictions.
French President Francois Hollande condemned the "scandalous" behaviour of the far-left activists.
"These disruptive elements have nothing to do with defenders of the environment," Hollande said at an EU-Turkey summit in Brussels.
Before flying out to Paris, Obama said world leaders would show their resolve to stand up to terrorism.
"It's an opportunity to stand in solidarity with our oldest ally, just two weeks removed from the barbaric attacks there, and reaffirm our commitment to protect our people and our way of life from terrorist threats," Obama said in a Facebook post before leaving Washington on Air Force One.
About 150 leaders, including China's Xi Jinping, India's Narendra Modi and Russia's Vladimir Putin, will attend the start of the UN conference that is tasked with reaching the first truly universal climate pact.
French Interior Minister Bernard Cazeneuve said nearly 1,000 people thought to pose security risks had been denied entry into France, which reimposed border controls on November 13 to protect the summit.
About 2,800 police and soldiers will secure the site of the November 30-December 11 conference, and 6,300 others will deploy in Paris.
The UN's weather body said this month the average global temperature for 2015 is set to rise one degree Celsius above pre-industrial levels, halfway towards the targeted UN ceiling.
Voluntary carbon-curbing pledges submitted by nations to bolster the Paris pact, even if fully adhered to, put Earth on track for warming of 2.7-3.5 degrees C, according to UN climate chief Christiana Figueres.
If concrete action is not taken soon, scientists warn of superstorms, drought and rising sea levels that will displace millions.
But this week's talks are set to see decades-long disputes between rich and poor nations flare again and potentially prevent an agreement.
Potential stumbling blocks range from finance for climate vulnerable and poor countries to scrutiny of commitments to curb greenhouse gases and even the legal status of the accord.
The last attempt to forge a global deal -- the ill-tempered 2009 Copenhagen summit -- foundered upon such divisions between rich and poor countries.
To pressure world leaders into putting aside their differences and forge an agreement, more than half a million people participated in global climate protests over the weekend, co-organiser Avaaz said.
"The charge from the streets for leaders to act on climate has been deafening, with record numbers turning out across the world," said Avaaz campaign director Emma Ruby-Sachs.