And he says member states should protect the legacy of the "founding fathers" as the EU celebrated the 60th anniversary of the Treaty of Rome.
Mr Tusk appeared to deride the UK for leaving the EU and called on remaining countries to get behind the bloc's failing policies.
The speech came as it emerged Greek politicians and those from Mr Tusk's own country Poland are causing ructions at the summit as leaders' gather to sign a new declaration.
Donald Tusk has called on member states to be unified but there's tensions in the bloc
Protect the legacy of the founding fathers
Polish Prime Minister Beata Szydło, who unsuccessfully tried to block the reappointment of Donald Tusk as European Council president, is also threatening to upset the apple cart.
He said: "I’m recalling history today to make everybody aware that for millions of people, EU is not about slogans and procedures and regulations.
Jean-Claude Juncker, EU president Antonio Tajani, Italy's PM Paolo Gentiloni, Donald Tusk and Malta'
"Our EU is a guarantee of freedom, dignity, democracy and independence.
"Our everyday reality, not only dreams.
"Today we are renewing a unique alliance initiated 60 years ago.
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Donald Tusk is meeting with EU leaders as they plan a new declaration
"Prove today that you are the leaders of Europe.
"Protect the legacy of the founding fathers".
British Prime Minister Theresa May is absent from the summit ahead of her triggering Article 50 of the Lisbon Treaty this week.
However summit leaders gathering in Rome to mark the 60th anniversary of the original treaty signing may not put their signatures to the new Rome Declaration text.
Leaders including Angela Merkel, Malta's Prime Minister Joseph Muscat, European Council President Donald Tusk, Estonia's Prime Minister Juri Ratas and Italy's Prime Minister Paolo Gentiloni have gathered at Rome's Piazza del Campidoglio to make their "declaration of intent".
Mr Tusk added: "Europe as a political entity will either be united, or will not be at all. Only a united Europe can be a sovereign Europe in relation to the rest of the world.
"And only a sovereign Europe guarantees independence for its nations, guarantees freedom for its citizens.
"The unity of Europe is not a bureaucratic model.
Merkel's plea for Tusk Thu, March 9, 2017
Merkel, who spoke ahead of a EU summit in Brussels, said the planned re-election of Poland's Donald Tusk as president of the European Union is a "sign of stability" for the bloc, despite bitter opposition from Warsaw
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French President Francois Hollande speaks with German Chancellor Angela Merkel on the second day of a European Summit at the Europa Building at the EU headquarters in Brussels
"It is a set of common values and democratic standards.
"Today it is not enough to call for unity and to protest against multiple speeds.
"It is much more important that we all respect our common rules such as human rights and civil liberties, freedom of speech and freedom of assembly, checks and balances, and the rule of law.
"This is the true foundation of our unity".