The European Union "will respond in kind," EU Council President Donald Tusk answered Donald Trump

EU leaders rounded on US President Donald Trump over his trade threats on Saturday as they arrived in the chic French seaside resort of Biarritz for a G7 summit overshadowed by trans-Atlantic tensions and worries about the global economy.

Already embroiled in a high-risk trade war with China, Trump warned late Friday he would impose punishing tariffs on French wine if France doesn't withdraw a new digital tax that will hit US tech giants.

The European Union "will respond in kind," EU Council President Donald Tusk answered to Trump, who issued his threat on departing Washington for Biarritz.

Host Emmanuel Macron, France's president, and even British Prime Minister Boris Johnson also sounded the alarm about the dangers of Trump's escalating trade war with China.

"I am very concerned. The UK is at risk of being implicated in this. This is not the way to proceed," Johnson told reporters on the plane to the G7 summit in Biarritz.

"I want to see a dialling down of tensions."

The stormy start to a summit of what are meant to be like-minded allies -- Britain, Canada, France, Germany, Italy, Japan and the United States -- led Tusk to sound a dire alarm over the future of Western leadership.

"This may be the last moment to restore our political community," he said. "The world needs our cooperation."

In an attempt to lighten the mood, Macron deployed his secret weapon of French cuisine diplomacy, treating Trump to a surprise lunch minutes after he had arrived on Air Force One.

Speaking to reporters in fluent English, Macron called Trump "a very special guest".

'So far so good'

Trump, sitting across the small table on a terrace of the ornate Hotel du Palais, appeared to be softened by the warm, unscheduled welcome.

"So far so good. The weather is perfect. Everybody's getting along. I think we will accomplish a lot this weekend," Trump said, praising his "special relationship" with Macron.

In addition to the global economy and fears of recession, the G7 chiefs will huddle over several hot topics, including their diverging positions on Iran's nuclear program.

European leaders are also focusing on a push for action against fires in the Amazon rainforest, despite Brazilian right-wing President Jair Bolsonaro's angry response to what he sees as outside interference.

Echoing criticism from France, Tusk said Bolsonaro's response to the "destruction of the green lungs of the Earth" was insufficient and he warned that a big EU trade deal with South America could be imperilled.

'People are mobilised'

G7 leaders were greeted by protests outside Biarritz, though 13,000 police have been deployed to keep them far from view.

Organisers said 15,000 people rallied around 30 kilometres (20 miles) south of the G7 gathering at the border town of Hendaye for a march over the Bidassoa River toward the Spanish town of Irun.

Red, white and green Basque flags waved above a crowd that included anti-capitalists, environmental activists as well as a few dozen of France's "yellow vest" anti-government protesters, according to AFP journalists at the scene.

"It's important to show that people are mobilised and do not accept the world they're offering us," said Elise Dilet, 47, of the Basque anti-globalisation group, Bizi.

The rally has been peaceful so far, after police said 17 people were arrested as of Friday night amid clashes with protesters camped out near Hendaye.

Escalating threats

Wall Street stocks slumped heavily Friday after Trump escalated his trade war with China.

"We don't need China and, frankly, would be far... better off without them," Trump tweeted on Friday.

His outburst came after China imposed tariffs on US imports worth $75 billion in response to an earlier round of American measures.

Trump hit back immediately, with a sharp increase in his own tariffs on all Chinese imports.

A Chinese commerce ministry spokesman on Saturday denounced Washington's "unilateral and bullying trade protectionism".

In a televised address ahead of the summit, Macron said his goal was "to convince all our partners that trade tensions are bad for everyone".

Tusk added: "Trade wars will lead to recession."

The G7 meeting marks the full international debut of Johnson, who will meet Trump for the first time as Britain's leader on Sunday.

They are expected to discuss the UK's impending exit from the European Union, which the US president has enthusiastically backed.

Johnson arrived saying that he was committed to yanking Britain from the EU with no deal on future relations if his conditions are not met.

But though Johnson needs Trump's support for a free-trade deal, he is at odds with him on a range of issues including the Iran nuclear crisis, climate change and global trade.