Activists gathered at the "Alternative G7" on the French-Spanish border want international deals such as the Paris Agreement on climate change to be legally-binding, in order to prevent nations from flouting their environmental commitments.
The counter-summit comes as world leaders — including Donald Trump, Boris Johnson and Emmanuel Macron — meet further up the coast at Biarritz.
Organisers of "Alternative G7", which has attracted around 4,000 activists so far, believe the countries of South America are not respecting accords such as Paris, which was signed by 195 nations in 2015. The US under President Donald Trump withdrew from the deal in 2017.
It comes as French President Macron called the record number of fires burning across Brazil's Amazon rainforest "an international crisis" and argued that solving it should be at the forefront of discussions at the G7 summit this weekend
Macron tweeted: "Our house is burning. Literally. The Amazon rain forest — the lungs which produce 20% of our planet's oxygen — is on fire. It is an international crisis. Members of the G7 Summit, let's discuss this emergency first order in two days! #ActForTheAmazon."
His tweet did not go down well with Brazil's president, Jair Bolsonaro, who accused Macron of seeking "to make personal political gains in an internal matter for Brazil and other Amazonian countries".
But Ireland's Prime Minister said his country will seek to block the free trade deal between the EU and Mercosur (South American Common Market) — unless Brazil takes action to protect the Amazon forest.
Despite his comments on Brazil, Macron is unpopular with activists in the border towns of Irun and Hendaye. "Alternative G7" organisers are boycotting both the summit and a meeting between the French president at members of civil society at the Elysee Palace Friday.
Among the dozens of groups represented at the counter-summit are representatives of the Yellow Vests, an anti-Macron movement that brought Paris to a standstill in mass protests last year. They are planning two demonstrations at the summit, including one close to Biarritz on Sunday.