"What doesn't kill you, makes you stronger," Tusk told reporters in a statement. "I want to reassure everyone that we are prepared also for this negative scenario," he said.
Tusk said there was no way to predict all the political consequences of the vote, especially for Britain, but that it is not a moment for "hysterical reactions."
"Today, on behalf of the 27 leaders I can say that we are determined to keep our unity as 27," Tusk said.
EU leaders, including British Prime Minister David Cameron, are scheduled to meet on Tuesday in Brussels. Tusk said the 27 leaders of countries remaining in the EU would meet informally to discuss how to handle Britain's decision to leave.