Sunday's presidential and parliamentary ballot will pass judgement on Turkey's longest-serving leader and the social transformation spearheaded by his Islamic-rooted AKP party.

The vote is Turkey's most consequential in generations and the toughest of the 69-year-old's tectonic career.

Polls show Erdogan locked in a tight battle with secular rival Kemal Kilicdaroglu and his powerful alliance of six parties that span Turkey's cultural and political divide.

The first votes were cast by Turks who moved from poorer provinces to Western Europe under job schemes aimed at combating the continent's labour shortage in the wake of World War II.

Such voters comprise 3.4 million of Turkey's 64.1 million registered electorate and tend to support more conservative candidates.

Official turnout on the morning of the last day of overseas voting on Tuesday exceeded 51 percent -- a touch higher than in the last general election that Erdogan won in 2018.

Kilicdaroglu's CHP has been trying to eat into Erdogan's traditional base of support by organizing daily buses to take voters to the Turkish consulate in Berlin.

Germany accounts for nearly half of Turkey's diaspora vote.

"It's not just a presidential election," opposition supporter Katresu Ergez said while waiting for a CHP bus.

"It's about voting for the future of the country, whether democracy will be restored or whether it will go further towards dictatorship," the 29-year-old said.