"We need to take seriously the fears of the people," former Belgian premier and Liberal hope Guy Verhofstadt said, referring to far-right parties playing the anti-immigration card in end-May European Parliament elections.
"All these problems need a European solution," Verhofstadt said during a debate organised by the Euronews channel in the Dutch city of Maastricht.
Socialist Martin Schulz, current head of the parliament, said it was not too late to tackle the problem, stressing "we need to have dialogue with citizens... We have to show we (have solutions)."
That "only strengthens them.... It is really dangerous to be afraid of confronting the far right," Keller said to rare applause.
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Juncker, former Luxembourg premier and backed by the European People's Party, the single biggest group in the current parliament, said "you have to make a difference between those criticising... And those against" European values.
"I will not debate with them," he said, adding that they were not "part of the European family."
"It is time you make your choice," he charged.