"The EU is one of the world's largest economic areas, a guarantor of peace and stability," she said.
"We must engage with the world. I believe the world will stand united."
Her words came after Tusk said he was "determined" to hold the European Union together, after the landmark result from yesterday's referendum, in which the UK voted for a Brexit.
In a brief statement this morning, Tusk reiterated that until the UK formally leaves the EU, its laws, rights and obligations will continue to apply.
"All the laws were set out in the treaties," he said.
|Brexit Britain: What you need to know|
He added that he had requested a meeting of leaders of the remaining 27 countries on 27 June - a day earlier than it was scheduled to take place.
"I have proposed we start a wider reflection on our union," he said.
"The past year has been the most difficult one in the history of our union. [But] my father used to tell me what doesn't kill you makes you stronger."
Tusk's comments came minutes before the opening of the UK stock market, and shortly before Prime Minister David Cameron was due to speak.
Bank of England governor Mark Carney will deliver a statement after Cameron. In a brief statement this morning the Bank said it had "undertaken extensive contingency planning and is working closely with HM Treasury".