The posturing by leaders of the G7 rich democracies began well before they stood together for a summit photo.

French President Emmanuel Macron, the host, cornered US President Donald Trump at his hotel with an impromptu lunch invitation.

It was something of a power move by Macron, who pushed the Amazon wildfires to the top of the agenda at a summit Trump hoped to use to persuade allies they can avoid economic disaster by following his low-tax, low-regulation mantra.

European Council President Donald Tusk and British Prime Minister Boris Johnson traded barbs over who would go down in history as "Mr No Deal" and take the blame for a Brexit departure from the European Union that threatens to go off the rails.

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And nearly everyone kept a trade threat close at hand.

Just before boarding Air Force One for France, Trump tweeted another threat of new tariffs on French wine in retaliation for France's digital services tax. Macron greeted him warmly as a "very special guest", but had already called for an end to the trade disputes that he said threatened global growth.

Tusk said the three-day summit in the seaside resort of Biarritz would be "a difficult test of the unity and solidarity of the free world and its leaders".

Tusk said the last thing the bloc wanted was a trade dispute with the US and called for "an end to trade wars", but promised to retaliate against US products if Trump carried through on the wine tariffs.

President Donald Trump and Britain's Prime Minister Boris Johnson, right, attend a working breakfast at the Hotel du Palais on the sidelines of the G-7 summit in Biarritz, France,. Photo / AP
President Donald Trump and Britain's Prime Minister Boris Johnson, right, attend a working breakfast at the Hotel du Palais on the sidelines of the G-7 summit in Biarritz, France,. Photo / AP

Macron has said the summit would not end with a final statement, as he had little expectation that Trump would agree to anything about fighting climate change even as the issue shot to the top of the agenda with the widespread fires in the Amazon. He already rejected Trump's request to let Russia rejoin the group five years after being expelled over its seizure of Crimea. And he is trying to hold together the European line on the Iran nuclear deal over US objections.

"We have disagreements, and at times there are caricatures. But I think that the great challenges that we have — climate, biodiversity, the technological transformation, the fight against inequality, this global insecurity — we will only resolve them by acting together, by reconciling," he said.

At last year's summit in Charlevoix, Canada, Trump left early and repudiated the final statement in a tweet from Air Force One.

This year, Macron said, there would be no final statement. Instead, diplomats say Macron could issue his own summary of the discussions.

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All eyes will be on the dynamic between Trump and Johnson, two figures who relish the unpredictability they have sown. Johnson is under intense pressure to pull Britain out of the EU and many see his relationship with the US as key.

Other leaders attending are German Chancellor Angela Merkel, Italy's caretaker leader Guiseppe Conte, Canadian leader Justin Trudeau and Japan's Shinzo Abe.

Well outside Biarritz, police fired water cannon and tear gas at about 400 anti-capitalist protesters blocking roads in Bayonne.

Earlier, thousands of demonstrators marched peacefully from the area to the Spanish border to demand more action against climate change and economic inequality.

- AP