Austria presidential vote: Postal ballots to decide result
Postal ballots will decide Austria's presidential election after polling station results from Sunday's vote gave the far-right candidate a slender lead.
Norbert Hofer of the Freedom Party led Alexander Van der Bellen by 51.9% to 48.1%, the interior ministry said on Sunday.
If elected, Mr Hofer would be the first far-right head of state in the EU.
A key campaign issue was Europe's migrant crisis, which has seen asylum-seeker numbers soar.
About 90,000 people claimed asylum in Austria last year, equivalent to about 1% of the Austrian population, and the Freedom Party ran an anti-immigration campaign.
Some 750,000 postal votes from roughly 12% of Austria's 6.4 million voters are being counted on Monday.
Mr Hofer has a margin of 144,006 votes, but some projections indicate the postal ballots could favour Mr Van der Bellen.
Analysis: Bethany Bell, BBC News, Vienna
Austria's election has revealed a profound split over which direction the nation should take.
Support for the Freedom Party has risen in recent years because deep frustration with the established parties of the centre left and centre right. Fears about the migrant crisis have boosted the far right still further.
The presidency is a mainly ceremonial post, but a victory for the Freedom Party candidate Norbert Hofer would give momentum to populist and far-right parties in other European countries.
Freedom Party leader Heinz-Christian Strache warned it would be "very questionable" if the postal ballots led to a Van der Bellen victory.
The presidents of the European Commission and the European Parliament, Jean-Claude Juncker and Martin Schulz, have both expressed concern over a Hofer victory.
The eventual winner would have "the job of uniting Austria", Mr Hofer said.
Mr Van der Bellen, an independent candidate who is supported by the Green Party, said he had championed the EU during his campaign.
For the first time since World War Two, both the main centrist parties were knocked out in the first round.
Vying to lead Austria
- Age: 45
- Background: Aeronautical engineer
- Politics: Far-right Freedom Party
- Campaign soundbite: "To those in Austria who go to war for the Islamic State or rape women - I say to those people: 'This is not your home'"
Alexander Van der Bellen
- Age: 72
- Background: Economics professor
- Politics: Former Green Party leader
- Campaign soundbite: "I've experienced how Austria rose from the ruins of World War Two, caused by the madness of nationalism."
What makes Norbert Hofer "far right"?
Mr Hofer's Freedom Party (FPOe) campaigns on a strongly anti-immigration and Eurosceptic platform, rejecting multiculturalism. The FPOe sits with other far-right parties in the European Parliament. Its roots lie in a pan-Germanic ideology, it claims to defend the values of the "fatherland" and the party's founder, back in 1956, was a former Nazi minister.
Some describe the FPOe as "right-wing populist" and its programme does speak of a commitment to "democracy", "freedom" and "the rule of law".
In recent years the FPOe has toned down some of its harsher rhetoric and "sought to appear more centrist", says political analyst Thomas Hofer, but "it's a far-right party, there is no question about that".
In a sinister echo of Nazi rhetoric about "blood", in 2010 FPOe General Secretary Herbert Kickl campaigned with the slogan "Viennese blood - too much foreign stuff doesn't do anyone any good".
And Heinz-Christian Strache has said "I am at heart with Pegida" - referring to the anti-Islam nationalist movement that has held regular marches in Germany.
Norbert Hofer has also said "I don't think Islam is a part of Austria", and he opposes same-sex marriage.
At his swearing-in as the FPOe's presidential candidate, Mr Hofer wore a cornflower in his lapel, which was a Nazi symbol in the 1930s.
He has also been photographed sporting the German colours of the nationalist Marko-Germania student fraternity, which stands for "the German cultural community" and bears the slogan "Honour, Freedom, Fatherland". It was founded in 1818.
What was the breakdown of the vote?
The election highlighted some sharp divisions in Austrian society.
In nine out of Austria's 10 main cities Mr Van der Bellen came top, whereas Mr Hofer dominated the rural areas, the Austrian broadcaster ORF reported (in German).
Support for Mr Hofer was exceptionally strong among manual workers - nearly 90%. The vote for Mr Van der Bellen was much stronger among people with a university degree or other higher education qualification.
Support for Mr Hofer among men was 60%, while among women it was 60% for Mr Van der Bellen.
What powers does the Austrian president have?
It is mostly a ceremonial post. But the president does have the power to dissolve the National Council - the more powerful lower house of parliament. That triggers a general election. The president can only do that once for a particular reason - he cannot use the same grounds to dissolve it again.
It is the chancellor's job to appoint government ministers. And the chancellor has the power to dismiss the government. But ministers have to be formally sworn in by the president. And Mr Hofer has said he would not swear in a female minister who wore a hijab, which he has described as a sign of oppression.