Voters in the German state of Berlin are casting ballots in a state election predicted to result in a fresh setback for Chancellor Angela Merkel's party.

Her Christian Democratic Union has been the junior coalition partner to the center-left Social Democrats in the capital for the past five years.

The city of 3.5 million has experienced a population boom during that time, putting pressure on housing, schools and its transport infrastructure.

The influx of tens of thousands of migrants over the past year has also become a campaign issue, though to a lesser degree than in other, less cosmopolitan parts of the country.

Pollsters forecast that the anti-immigrant Alternative for Germany will easily pass the 5-percent threshold to enter the Berlin state assembly for the first time, its 10th nationwide.