Finland's center-left Social Democratic Party is facing the tough task of trying to form a government coalition after becoming the Nordic country's largest party by an extremely narrow margin following Sunday's highly fragmented parliamentary election.

With all votes counted Monday, the party led by Antti Rinne took 17.7% of votes and 40 seats in the Eduskunta legislature, far from the 101 seats needed for a majority.

Right behind came the populist Finns Party with 17.5%, while the conservative National Coalition party took the third spot with 17% of votes. Outgoing Prime Minister Juha Sipila's Center Party was fourth with 13.8 %.

Tackling climate change and reforming Finland's social and health care system were key topics in the vote where established parties lost support to populists in line with an overall European trend.

Mr Rinne, the Social Democrat leader, is a former trade union boss once known as the "labour market gangster" for his fiery negotiating skills.

Aged 56, the silver-haired and serious-looking Rinne is no stranger to the top of government, having become finance minister in 2014 when he deposed the then leader of his party, Jutta Urpilainen, in a bitterly fought leadership contest.

Mr Rinne had strongly opposed Urpilainen's decision to support the spending cuts put forward by the then government coalition's largest partner, the centre-right National Coalition Party.