German Chancellor Angela Merkel's party is set for further losses in regional elections in the capital Berlin on Sunday, with the right-wing populist Alternative for Germany (AfD) party expected to make gains.

Support for Merkel and her conservative Christian Democrat Union (CDU) party has been hit by her open-door migrant policy, with a record number of people moving to Germany in 2015, according to official statistics.

This is expected to tell in Sunday's elections, which come after the CDU was knocked into third place by the AfD in regional elections in the region of Mecklenburg-Vorpommern earlier this month.

Voting in Berlin started at 7 a.m. London time Sunday and polling stations will close at 5 p.m. London time, according to Reuters. Public broadcasters will publish exit polls shortly afterwards.