The bloc's leaders will hope to present a united front against Mr Trump in response to his decision on the Iran nuclear deal and trade tariffs.
The European Union's top official launched a stinging attack Wednesday on President Donald Trump, slamming his "capricious assertiveness" and saying the USA leader acted more like an enemy than a friend.
The senior official said that European Union should be grateful to Trump as he helped the bloc get rid of its illusions.
European leaders will gather in Sofia to mainly discuss the EU's commitment towards enlargement talks focused around Western Balkan regions, but the US President's actions have seemingly hijacked the agenda.
European ministers met with a top Iranian official in Brussels on Tuesday in a bid to save the Iranian nuclear accord after Trump made a decision to pull out.
Trade tensions have been growing since March 23, when Donald Trump ordered to introduce a 25% tariff on steel imports and 10% tax on aluminum, claiming to protect the U.S. producers from unfair competition and boost national security.
In a scathing attack, former Polish prime minister Tusk compared the Trump administration with Europe's long-time adversaries, Moscow and Beijing.
Mr Tusk said: "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 American administration".
"I have no doubt that in the new global game, Europe will either be one of the major players, or a pawn".
Tusk said the European Union must be more united than ever before to face off against Trump's "capricious assertiveness".
President of the European Commission Jean-Claude Juncker also remarked last week that the European Union should replace the United States as a global leader after the latter "as an worldwide actor has lost vigor, and because of it, in the long term, influence".
Theresa May will travel to the Bulgarian capital where she will join a working dinner with her fellow European leaders, including European Commission President Jean-Claude Juncker.