Speaking in Berlin on the 26th anniversary of the fall of the Berlin Wall, Tusk described Germany and Merkel personally as examples of the best European values.
"Those who believe that Germany is too open, too tolerant, too liberal, forgot to do their homework about our tragic history," said Tusk, a former Polish prime minister.
"Do you want a Germany that is open, tolerant, compassionate, sympathising with the weaker and the poorer, in other words the Germany of Angela Merkel, or a Germany which is closed, cold and ruthless? There is only one answer," he added.
Therefore, other European states should now show solidarity towards Germany "in these difficult and testing times", he said.
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EU chief executive Jean-Claude Juncker has accused national leaders of sapping efforts to tackle the migration crisis by not honouring commitments on money and resources.
Germany, on the other hand, should realise that it is responsible not only for "its moral message", but even more so for the whole political community of Europeans, Tusk added.
He urged Germany to provide strong leadership by helping to secure Europe's external borders and protect Europe against a rise of radical populism.
"Indeed, whether Europe survives as a continent of freedom, the rule of law, respect for an individual, and the security of its inhabitants will depend to a great extent on Germans."
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Tusk called for a "modification" of the current European migration policy, warning that Europe's Schengen zone of passport-free travel was at risk if external borders were not strengthened.
"In the face of the unprecedented scale of migrants flowing to Europe, we have to say in simple terms: Europe is not able to accept all the people willing to come to our continent."
"Let us not fool ourselves. The fall of the Berlin wall did not automatically abolish the need for borders as such."
Since Germany is not a European border state, responsibility lies in the first place with other countries, Tusk said. "But even so, everybody will be looking up to you, watching out for signals coming from Berlin," he added.
Merkel has been criticised for unwittingly encouraging more refugees to come to Germany by stating publicly that there was no upper limit to the number that would be accepted.
EU leaders hold talks with African leaders on controlling migration on Wednesday and Tusk has called an informal meeting of EU heads of state on the refugee crisis on Thursday.