Protesters converged on the nearby town of Hendaye on the French border with Spain to protest against economic and climate policies pursued by the world's leading industrial nations and to promote alternatives.
"The top capitalist leaders are here and we have to show them that the fight continues," said Alain Missana, 48, an electrician wearing a yellow vest -- symbol of the anti-government demonstrations that have been held in France for months.
"It's more money for the rich and nothing for the poor. We see the Amazonian forests burning and the arctic melting. The leaders will hear us," he said.
Fires are devastating large swathes of the forest which is considered a vital bulwark against climate change.
Protesters waved banners for causes ranging from gay rights to Palestine, but their messages were aimed firmly at the leaders of the United States, Germany, France, Britain, Canada, Japan and Italy who are set to begin three days of talks on Saturday.
"No to the G7. For another world," one banner read. "Heads of state the Amazon is burning. Act now," said another.
The protesters marched under bright-blue summer skies from Hendaye to the town of Irun, Spain, some 30 km (18 miles) south of the G7 venue Biarritz.
More than 13,000 police officers, backed by soldiers, are guarding the Biarritz summit site and police had feared that anarchist groups might have tried to derail Saturday's protest, which has been billed as a peaceful, family event.
Four police officers were lightly wounded on Friday after protesters fired a homemade mortar near the anti-G7 counter summit in Hendaye. Police arrested 17 people for hiding their faces.
There was no immediate sign of any radical groups on Saturday and the police presence was light.
Protesters came from all parts of France and beyond including from across the border, where Basque separatists were also keen to show their solidarity.
"The counter-G7 demonstration is in this Basque region and we want people to see we are part of it," said Alfredo Akuna, a 46-year-old engineer from San Sebastián who wore traditional Basque clothing.
"We're involved in many movements including anti-capitalism and anti-fascism so it's important to be here to show that."