In comments to reporters, Mr Macron signalled Europe would not surrender meekly to the US president and suggested the G7 could function as six nations instead of seven.
“Maybe the American president doesn’t care about being isolated today, but we don’t mind being six, if needs be,” Mr Macron said.
“Because these six represent values, represent an economic market, and more than anything, represent a real force at the international level today.”
Justin Trudeau, Canada’s prime minister, predicted “robust discussions” on trade. The Canadian leader is also embroiled in a row with Washington over negotiations to revamp the almost 25-year-old North American Free Trade Agreement (NAFTA).
Other issues which are likely to be raised include Mr Trump’s decision to take the US out of the Iran nuclear accord and pull out of the Paris climate agreement.
However, Mr Trump fired back at the two leaders over Twitter, accusing the EU and Canada of imposing “massive trade tariffs on non-monetary trade Barriers” against the US, and he threatened to take retaliatory action against the bloc and country unless the measures were removed.
He said: ”Why isn’t the European Union and Canada informing the public that for years they have used massive Trade Tariffs and non-monetary Trade Barriers against the US.
“Totally unfair to our farmers, workers & companies. Take down your tariffs & barriers or we will more than match you!”
He added: ”I look forward to seeing them tomorrow.”
Before flying to Canada for the summit on Friday morning, Mr Trump injected fresh controversy by suggesting Russia should be let back into the G7. Moscow had been part of what was formally the G8, but was suspended over the annexation of Crimea from Ukraine in 2014.
“Why are we having a meeting without Russia in the meeting?” Mr Trump asked outside the White House. “They should let Russia come back in because we should have Russia at the negotiating table.”
Whether intentional or not, Mr Trump’s remarks pushed talk of trade into the background at least for a few hours, with Italian Prime Minister Giuseppe Conte tweeting his support for the idea, while both Mr Trudeau and UK Prime Minister Theresa May suggesting that countries should remember why Russia was ousted in the first place.
Other G7 members appeared less likely to want to challenge the US president, with Japan being expected to take a less confrontational approach while quietly pressing its case on trade, and Ms May urging the EU to stick to the World Trade Organisation (WTO) rules and to ensure any response to the tariffs was proportionate.
The British prime minister will hold formal meetings with the leaders of all G7 countries except Mr Trump. But she is expected to have informal talks with the US president.
On the plane to the summit in Quebec, Ms May told reporters: ”I made my views clear on the steel and aluminium tariffs that President Trump has announced, I have done that directly to him.
She added: “As the UK, we want to be a great champion of free trade around the world and that’s what we will continue to be.
“I will continue to put the argument for the importance of those trade relationships around the world and I’ll be doing that here at the G7 as I have done elsewhere and will continue to do elsewhere.”
Mr Trump’s approach to the G7 summit represents an abandonment of America’s traditional role.
Previous presidents have lobbied for freer global trade and championed a trading system which required countries to follow WTO rules.
By contrast, Mr Trump’s policies are unapologetically protectionist and confrontational.
The US president is expected to leave the summit in Charlevoix, Quebec, early to fly to Singapore for his historic meeting with the leader of North Korea, Kim Jong-un, on Tuesday.
The Independent with additional reporting by agencies