Finns head to the polls on Sunday to cast their ballots in a first round of presidential polls following a campaign overshadowed by the eurozone crisis, even though the president no longer decides EU affairs.

Pro-European Sauli Niinistoe, of Prime Minister Jyrki Katainen's conservative National Coalition Party and one of Finland's most experienced politicians, has enjoyed a substantial but slipping lead in the polls.

The 63-year-old former finance and justice minister has seen 50 percent support slump, according to the most recent survey, to just 32 percent.

While he is no longer expected to win Sunday's vote with an outright majority, meaning a second round of voting on February 5 will be needed, he remains well ahead of his closest rivals.

Pekka Haavisto, the 53-year-old EU-friendly Green candidate, has surprisingly soared into second place in recent weeks from single digit obscurity to 13 percent in the latest survey, while 65-year-old eurosceptic Paavo Vaeyrynen of the traditionally agrarian Centre Party, has slipped to third place with 11 percent of voter intentions.

Populist and deeply eurosceptic Finns Party candidate Timo Soini, 49, has meanwhile seen his support dwindle from double digits to six percent, but has continued to colour the debate with his virulently anti-euro rhetoric.

The president and government jointly run Finland's foreign policy. However, in October parliament voted to hand EU policy to the government, with the prime minister now the main representative in EU affairs.

Despite that change, the euro issue has dominated the campaign because it remains "pretty close to the hearts of voters," Helsinki University political science professor Tuomo Martikainen told AFP.

Finland, with its 5.3 million inhabitants, is in an elite club of eurozone members with a triple-A credit rating and is key to any bailout deals.

Niinistoe -- instrumental in leading Finland into the eurozone during his tenure as finance minister from 1996-2003 -- has so far withstood the fiery debate on Finland's role in the euro club.

Out of the Finland's 4.4 million eligible voters, 1.3 million had already cast their ballots in advance voting before the polls stations open at 9:00 am (0700 GMT) Sunday.

Polling stations close at 8:00 pm (1800 GMT) and final results of the first voting round are expected around two hours later.