He said Europe didn't need enemies when it had friends like the United States.
European diplomats have sought to play down expectations of Tuesday's meeting, stressing the enormous challenge of finding a way around USA sanctions punishing foreign businesses trading with Iran, which have global reach.
"I want the European Commission to be given a green light, so that it is ready to act whenever European interests are affected", he said.
The break with the USA over the Iran deal is just the latest in a series of major breaches in the transatlantic relationship.
That came a day after many European newspapers featured dueling photographs on their front pages: the beaming visage of first daughter Ivanka Trump opening the new U.S. Embassy in Jerusalem, and the smoky carnage an hour's drive away in Gaza, where scores of Palestinian protesters were killed by the Israeli military.
Tusk also demanded that Trump grant a permanent exemption to the European Union from threatened tariffs on steel and aluminum, and he rejected as "absurd" any assertion by the that such tariffs could be justified on national security grounds.
"I will propose we stick to our guns", Tusk said.
The European Commission is refusing to open talks on wider trade terms with the USA, including import tariffs on cars - a bugbear of the United States president - until it receives a permanent exemption from punitive taxes on European steel and aluminium. The EU and U.S. are friends and partners, therefore United States tariffs can not be justified on the basis of national security'. "We need to bring reality back into this discussion".
While European officials have spoken out against Trump's trade actions before, the language used by Tusk marks a new tone in the relationship.
The EU and the United States have traditionally been the closest of allies, working together also via North Atlantic Treaty Organisation.
"But frankly speaking, Europe should be grateful for President Trump because thanks to him we have got rid of all illusions. We realize that if you need a helping hand, you will find one at the end of your arm", Tusk said in Sofia, also posting the comments in a Tweet tagging the U.S. president.
"Besides traditional political challenges, such as the rise of China or the aggressive stance of Russian Federation, we are witnessing today a new phenomenon - the capricious assertiveness of the USA administration", Tusk said.