The bloody carnage at a Berlin Christmas market, with a Pakistani asylum-seeker as an early suspect, immediately emboldened right-wing populist critics of Chancellor Angela Merkel's liberal refugee policy.

Merkel has been under fire over her decision in September last year to let in tens of thousands fleeing war and poverty from mostly Arab and Muslim nations.

The arrival of around a million people over the past two years has deeply polarised society, and the German leader has said that next year's election would be "more difficult" than any other she has contested.

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