VIENNA - Austrian far-right hopes of winning a presidential runoff remained on hold Sunday as the candidates were neck and neck in a nail-biting race closely watched by the EU, which is struggling to contain a surge of anti-immigrant parties.

A win would see Norbert Hofer of the Freedom Party (FPOe) become the European Union's first far-right head of state.

On Sunday evening, the vote was evenly split between Hofer and Green-backed economics professor Alexander van der Bellen, with both on 50.0 percent according to projections after all the ballots cast Sunday had been counted.

This means the winner will only be known on Monday once the tie-breaking postal ballot has been added.

Close to 900,000 people - or a record 14 percent of Austria's 6.4 million eligible voters - cast their ballot by mail this year.

"Neither of us wished for this, we both wanted to sleep well tonight," Hofer told public broadcaster ORF, adding "it was not an easy" battle.

Ex-Green party leader van der Bellen meanwhile thanked voters for helping him achieve a remarkable comeback after Hofer comfortably beat him by 14 percentage points in the first round.

"Few people believed that I could catch up. But in the last 14 days there has been such a momentum among voters - musicians, actors, workers, totally different people across all generations, professions and all sections of society," he said.

Political analyst Karin Cvrtila told AFP the surprising result and high turnout had transformed the race into a "real election thriller", which showed "people were alarmed" at the idea of Hofer becoming president.

Voter turnout was expected to be more than 70 percent.

- 'I am not dangerous' -

A huge influx of asylum seekers brought on by the migrant crisis, rising unemployment and frozen reforms have driven voters away from the two centrist parties that have dominated Austrian politics since 1945.

They are being forced to watch the battle between Hofer and van der Bellen from the sidelines after their candidates failed to make it through a first round for the largely ceremonial post of president.

Instead, disgruntled Austrians flocked to the FPOe's "friendly face" Hofer who has pushed populist themes with a winning smile instead of the inflammatory rhetoric used by party leader Heinz-Christian Strache.

But observers have warned that beneath the smooth image lurks a "wolf in sheep's clothing".

He has already threatened to seize upon never-before-used presidential powers and fire the government if it fails to get tougher on migrants and boost the faltering economy.

But gun enthusiast Hofer, who was left partially disabled after a paragliding accident, insisted Sunday he was "not a dangerous person".

"I am a centre-right politician with a great social duty," he told reporters.

Hofer's win would also pave the way for the FPOe to head the next government after parliamentary elections, scheduled for 2018.

- 'Pest and cholera' -

A victory for the far-right is bound to send shockwaves across the crisis-hit European Union.

While the FPOe's allies including France's National Front will rub their hands in glee, the prospect represents a major headache for EU leaders.

Ahead of the vote, EU Commission President Jean-Claude Juncker warned "there will be no debate or dialogue with the far-right".

Back in 2000, more than 150,000 people marched in the Austrian capital against the FPOe - then led by the late, SS-admiring Joerg Haider - after it entered a much-maligned coalition with the centre-right.

This also led to international isolation and turned Austria into an EU pariah.

But times have changed, with eurosceptic and populist parties now posing a serious threat to traditional centrist governments.

In Austria - the receiver of some 90,000 asylum requests last year - the main parties have been haemorrhaging support to the FPOe, which consistently scores more than 30 percent in opinion polls.

The demise means the Social Democrats (SPOe) and centre-right People's Party (OeVP) could fall short of being able to re-form their "grand coalition" at the next scheduled election in 2018.

In the last vote three years ago, they only just managed to secure a majority.

Although van der Bellen enjoyed backing from many public figures including new Chancellor Christian Kern, he has been a divisive figure, with conservative Austrians accusing him of pandering to the left.

"It's a choice between pest and cholera. Whoever wins, I will wake up on Monday to somebody whom I don't want to represent Austria," said a mother-of-two in her thirties, refusing to give her name, after she cast her vote in Vienna.