"I sometimes get the impression that the Russians better understand the importance of these elections than some of us," said Tusk in a campaign speech urging Poles to vote.

He warned against "radical-right candidates" he said would "stoke sentiment that is anti-Ukrainian, anti-European and increasingly pro-Russian.

"We can't leave Poland and Europe in the hands of lunatics and traitors."

Russian and Belarusian services were trying "more actively" to influence the course of events in Europe, said Tusk.

"We have more and more evidence that the Russians will want to interfere in the (European) electoral process in various ways, including via violence," he warned.

The European Union had to become a fortress against "aggression, chaos and disorder", he argued, referring to the war across the border in Ukraine, where Kyiv is fighting Russian forces.

Arguing for greater European cooperation on security matters, he added: "We need to effectively defend our sky, our territory and our borders against aggression, chaos and disorder."

Tusk has already said Poland plans to join Germany's air-defence project, called the European Sky Shield Initiative, to which some 20 countries have signed up.

Tusk has repeatedly stressed the importance of European security.

In March, he warned of the "real" threat of conflict in Europe, saying that for the first time since the end of World War II, the continent had entered a "pre-war era".

Earlier this month, he urged the EU to think more actively on how to protect countries like his from fall-out from the Ukraine war.