"The only way to stop Russia and also prevent tragic conflicts in this part of the world is through firm pressure and obstinacy on the part of Europe, the United States and Canada," Tusk told reporters.
Several hundred people protested in Warsaw brandishing signs comparing Russian President Vladimir Putin to Hitler and Stalin, while waving Polish, Ukrainian, EU and Belarusian flags.
"Don't Touch Ukraine", "Don't Touch Crimea" and "Free Ukraine", they chanted.
Hundreds also protested peacefully in front of the Russian embassies in Berlin and in London.
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British-based Ukrainians held up banners demanding "Hands Off Ukraine" and chanted "Stop the Occupation" and "Putin Is a Murderer".
In Berlin, around a dozen protesters lay down in front of a poster listing past Russian military interventions and ending with the question "Ukraine in 2014?".
British Foreign Secretary William Hague was headed to Kiev today for talks with the interim pro-Western government.
In Warsaw, Tusk called for a "calm, balanced but also firm and united policy against Russia, based on disapproval of these kinds of actions and announcements by Moscow... To avoid a major conflict.