"Please tell Prime Minister Trudeau and President Macron that they are charging the USA massive tariffs and create non-monetary barriers", he tweeted.
Senate Democratic leader Chuck Schumer criticized the Republican president's leadership.
The US president had earlier called for Russian Federation - which was suspended from the group in 2014, following Moscow's annexation of Crimea, and made a decision to leave permanently in 2017 - to be readmitted in the interests of realpolitik.
Italy's new prime minster, Giuseppe Conte, agreed with Trump's take.
Instead of a G6-plus-one summit, it was now a G5-plus-one-plus-one meeting.
Before the president arrived at the summit, he criticized Canadian tariffs on US dairy products, adding that he was "looking forward to straightening out unfair Trade Deals with the G-7 countries". "And then we'll see what happens".
In response Canada implemented retaliatory tariffs on US goods.
Trump was referring to the retaliatory tariffs Canada slapped on the US on a number of items totaling about $16.6 billion worth. The United States sells more dairy products to Canada than it buys in return. Then he pulled the USA out of the Paris accord the following week.
Foreign Affairs Minister Chrystia Freeland held firm against what she called "illegal and absolutely unjustified" tariffs even though the leaders were playing nice.
Relations with Russian Federation also became an issue at the summit after Trump on Friday said the country should be allowed to again attend meetings with the G7, an idea that did not gain much traction at the meeting. The U.S. and the European Union had been contemplating just such a deal, called the Transatlantic Trade and Investment Partnership, or TTIP, but it was abandoned after Trump won the 2016 election.
Officials said European Commission President Jean-Claude Juncker offered to visit Washington to continue addressing Trump's concerns, but that Trump had not responded to the suggestion. Mr. Trump is a constant surprise.
German Chancellor Angela Merkel on Friday floated an idea to set up a way to resolve trade disputes between the United States and its allies, a French official said.
"Let's leave seven as it is", Tusk said.
You can bet Mr. Tusk's list of the usual suspects out to disrupt the world order includes Russia's Mr. Putin.
When Trudeau complained to Trump about the new tariffs he slapped on Canadian goods, saying he was especially upset that the pretext Trump used was that Canada formed a "national security threat" to the US, Trump replied, "didn't you guys burn down the White House?" He has also refused to condemn the Russian strongman over the Kremlin's interference in the USA presidential election, at one point suggesting he trusts Putin's denials of wrongdoing over the findings of his own intelligence agencies.
Chinese President Xi Jinping and British Prime Minister Theresa May have also been spurned by the president despite chummy beginnings. Mr. Trump's United States doesn't worry about allies.