(Corrects paragraph 4 to restore dropped word 'million' in number of Turkish citizens in Germany.)
By Daren Butler
ISTANBUL (Reuters) - Turkish citizens based abroad began voting on Saturday in Turkey's presidential runoff election between the incumbent Tayyip Erdogan and his challenger Kemal Kilicdaroglu, who aims to bring an end to the president's two-decade rule.
The runoff election will be held in Turkey on May 28 after Erdogan fell just short of the 50% threshold needed to win the presidential vote outright last Sunday in what had been expected to be his greatest ever political challenge.
Some 3.4 million Turks are eligible to vote abroad, out of a total electorate of more than 64 million, and will cast their ballots from May 20-24.
State-owned Anadolu news agency said voting had started in countries across Asia and Europe. Germany is home to the world's largest Turkish diaspora, where there are some 1.5 million Turkish citizens eligible to vote.
In last Sunday's vote, Erdogan's ruling AK Party and its nationalist allies won a comfortable parliamentary majority.
Kilicdaroglu, candidate of a six-party opposition alliance, won 44.88% support in the presidential election, trailing Erdogan on 49.52% and confounding expectations in opinion polls that the challenger would come out ahead.
Attention is now focused on nationalist Sinan Ogan, the candidate who came third with 5.17% support. Any decision by him to support one of the two candidates in the runoff could potentially have a decisive role.
Kilicdaroglu's rhetoric has taken a nationalist turn after he trailed Erdogan in the first round of voting, saying that the government had allowed 10 million refugees into the country and that he would repatriate them all if he were elected.
He provided no evidence regarding the number of migrants. Turkey hosts the world's largest refugee population of around 4 million, according to official figures. Ogan had campaigned on sending back migrants, including some 3.6 million Syrians displaced by war to the south.
Erdogan says only he can ensure stability in Turkey, a NATO member state, as it grapples with a cost-of-living crisis, soaring inflation and the impact of devastating earthquakes in February.
(This story has been corrected to restore dropped word 'million' in the number of Turkish citizens in Germany, in paragraph 4)