European Commission President Jean-Claude Juncker is urging people to carefully weigh their votes in the upcoming EU elections as polls show that populist and nationalist parties could gain ground.

Juncker said Tuesday that voters should ask themselves what the "European landscape" would look like in the coming years if everyone voted for extremists.

He underlines that the May 23-26 ballot to choose members of the European Parliament "is the largest transnational election anywhere in the world."

Juncker slammed political attacks on EU Council President Donald Tusk comparing him to Hitler and Stalin as "totally unacceptable, I have to say, highly disgusting."

Polls suggest mainstream parties will lose some support but still hold control over the assembly while fringe groups are likely to win more seats.