Since Monday, anti-capitalist activists, environmentalists and other anti-globalisation groups have been flocking to a counter-summit in southwestern France that organisers insisted would be peaceful.
Saturday's march took place in the French coastal town of Hendaye, about 30 kilometres (18 miles) from Biarritz, without incident in a good-natured atmosphere, with police giving a figure of 9,000 but organisers saying as many as 15,000 people turned up.
Biarritz is a popular tourist destination that would normally be basking in its annual summer boom, but with US President Donald Trump and other world leaders flying in for three days of talks, the resort was in lockdown.
"Heads of state: act now, Amazonia is burning!" read one banner as the huge crowd rallied under cloudless blue skies in Hendaye, the slogan referring to the wildfires ravaging the world's largest rainforest.
"If the climate was a cathedral, we would already have saved it," read another, referring to Notre-Dame in Paris, which was ravaged by a fire in April that prompted donors to pledge 850 million euros ($954 million) to rebuild it.
Waving thousands of flags, they marched across the Bidassoa River heading for the Spanish town of Irun, chanting slogans while some played drums.
The colourful crowd was an eclectic mix of environmental activists, families, anti-globalists, a handful of anti-government "yellow vest" protesters and Basque nationalists, AFP correspondents said.
There was even a group dressed in traditional Basque shepherd costumes, with red, white and green Basque flags as far as the eye could see.
"We are very happy because it was a huge challenge," said Sebastian Bailleul of Alternatives G7, one of the march's organisers, saying the event had "brought together French, Basque and international struggles".
But authorities remain on high alert, with Biarritz in lockdown and police deployed en masse in the neighbouring town of Bayonne as well to keep protesters at bay.
"I want to call for calm and for unity," French President Emmanuel Macron said in an address to the nation just before the opening of the summit, where world leaders were to address the Amazon crisis along with other global issues.
"We won't be able to face all these big challenges if we don't act together," he said.