Queen Elizabeth II before heading to Scotland. He will cap it off with a summit on Monday with Russian President Vladimir Putin in Helsinki.

Trump has previously attacked NATO, the military alliance of 29 countries across North America and Europe formed in the aftermath of World War II, for shortchanging the U.S. and vowed to upend the crucial relationship if other member countries did not contribute more.

Donald Tusk, president of the European Council, responded to Trump's claims on Tuesday, criticizing the president for attacking its allies.

"America does not have, and will not have a better ally than Europe," Tusk said at a joint E.U.-NATO declaration Tuesday. “Dear America, appreciate your allies, after all you don’t have all that many."

Tusk noted that Europe has promised to increase common defense spending,but noted that the E.U. contributes more than Russia and as much as China.

Trump’s visit is expected to draw large protests in the U.K. Tens of thousands of demonstrators are expected to march in London, Scotland and elsewhere during his trip amid growing trade tensions between Europe and the U.S. and objection to the president's immigration policies.

Before departing for his trip on Marine One on Tuesday, Trump continued to criticize the E.U. for its NATO contributions and reacted to Tusk's comments about the scarcity of America's allies.

"Well, we do have a lot of allies, but we cannot be taken advantage of, we are being taken advantage of by the European Union," Trump said, adding that NATO "helps them a lot more than it helps us."

He also dismissed criticism