Mr Trump departed before the other leaders, and complained that America was "like the piggy bank that everybody is robbing".
Beside Trump is his national security adviser, John Bolton, apparently speaking above the locked eyes of the USA and German leaders. Trump said the tariffs are necessary for national security.
Mrs Merkel said during the summit that she and Mr Trump did not always agree, but that they could talk to each other: "I can say that I maintain a very open and direct relationship with the American president". Despite this, they were seen at the summit on good terms, and President Macron's team said his talks with Trump were "frank and robust".
Soon afterwards, Trump rejected in a series of angry tweets from Air Force One the text of a G7 consensus statement - traditionally a paean to shared Western values and objectives under American leadership.
Prior to the summit Mrs May had called the tariffs "unjustified and deeply disappointing".
The other G7 leaders met without Trump for the rest of the day, discussing climate change and worldwide development.
Merkel captioned the picture: "Day two of the G7 summit in Canada: spontaneous meeting between two working sessions".
The photo was tweeted by Merkel spokesman Steffen Seibert.
An official White House photograph from the summit shows Mr Trump holding forth as Mrs Merkel, Canadian Prime Minister Justin Trudeau and Prime Minister Shinzo Abe of Japan listen.
Yet after leaving the summit, Mr Trump tweeted that he would instruct United States officials not to endorse the G7 statement, after objecting to comments from summit host Mr Trudeau.
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