Sweden faces weeks or months of talks to form a new government after the election ended with its two main centrist coalitions failing to win a majority and the far-right Sweden Democrats making gains over immigration fears.

Prime Minister Stefan Lofven, head of the country's centre-left bloc, called for cooperation across the political divide amid concerns the centre-right opposition may attempt to form a government with the help of the Sweden Democrats, a party with roots in the neo-Nazi movement.

At a rally after the polls had closed, Lofven said he would remain in office in the coming weeks and asked people not to speculate on the election's outcome.

“We have two weeks left until parliament opens. I will work on calmly, as prime minister, respecting voters and the Swedish electoral system,” Lofven told a party rally.

Lofven's bloc maintained a slight lead with 40.6 percent of the vote, giving them 144 seats, with the centre-right alliance winning 40 percent, or 143 seats, with 175 needed to form a majority in the 349-seat Swedish parliament.

However, the unaligned far-right Sweden Democrats gained 17.6 percent of the vote, or 62 seats, up from 12.9 percent in the previous 2014 elections. Results were based on 99 percent of the vote confirmed, and will not be finalized until Wednesday when overseas votes are counted.

Claiming a kingmaker role for this party, Jimmie Åkesson, leader of the Sweden Democrats, said in a speech at his campaign headquarters that his party will now have “influence over Swedish politics.”

The Sweden Democrats want to freeze migration and have pushed for the country to leave the European Union. Although the party did not fulfil exit polls predictions that it would become Sweden's second-biggest party, its success has confirmed fears that Europe's rising far-right tide has now reached Sweden, one of the world's most liberal countries.

Both of Sweden's main blocs have pledged not to work with the Sweden Democrats, but political scientist Nicholas Aylott said that factions of the centre-right alliance might want to reach an accommodation with it.

Immigration became highly politicized following a steady increase in new arrivals. In 2015, Sweden, a country of 10.1 million people, took in more than 160,000 asylum-seekers.

Support for the far right in Sweden mirrors similar trends in other European nations following the mass migration of refugees to the region in 2015, at the height of the Syrian war. Anti-migrant parties in Germany, Austria, Denmark, France, Hungary, Italy and the UK have all made gains in recent years.

Senior Social Democrats official Anders Ygeman conceded that the crisis had damaged his coalition's prospects. “We paid the price for being in government. We suffered from the refugee crisis in 2015. That's why the Sweden Democrats are as big as they seem to be,” he told CNN in Stockholm.

The results show Sunday's vote was one of the toughest challenges in decades to Sweden's social democracy, characterized by high tax rates and a substantial welfare system, aimed at reducing inequality through social inclusion.

If the ruling coalition fails to form a government it would be a historic loss for the Social Democrats, which have dominated Swedish politics since the 1930s.

In Sunday's election, the Social Democrats won the largest share with 28.4 percent of votes, giving them 101 seats, with the centre-right Moderate party taking 19.8 percent, or 70 seats.

But the two main parties lost the most ground compared to 2014 elections, with the Social Democrats losing 12 seats and Moderate party 14. The populist nationalist Sweden Democrats gained 13 seats.

In his post-election speech, Sweden Democrats leader Åkesson called on other parties to acknowledge his party's success by entering into talks with it. He singled out the centre-right's candidate for premier, Moderates leader Ulf Kristersson, for possible negotiations.

“I invite Ulf Kristersson to talk about how this country is supposed to be governed in the future,” he said.

“This is a very clear signal to the voters that the government we have had, it does not support the support of the majority of voters,” he added. Sweden has had a succession of the recent coalition and minority governments, but like in many European elections, the vote has been further splintered, making coalition talks likely to be torturous.

Political analyst Aylott said that while a German-style “grand coalition” between the main Social Democrats and Moderates is possible, another alternative might be an alliance under a new prime minister chosen as “the candidate parliamentarians dislike the least” in accordance with Sweden's parliamentary system.

“Quite possibly, then, Sweden will emerge with a rather extreme form of minority government,” said Aylott. “It might not be what Sweden really needs. But it is not a recipe for chaos.”

Voter Anton Loin said Swedes were increasingly looking for political alternatives. “I think that it shows that people are disappointed with how the country is run, and they are rooting for something, they want something different, but it's not necessarily the best kind of different,” he said of the far right's rise.

Tuva Sundh, who also cast her vote Sunday, said she was concerned about integration. “But I'm not sure about the way of the Sweden Democrats. I think it's healthy to have a debate. But I do think it's become too heated in this election.”

Leaders in Brussels will be disappointed with the party's surge ahead of the European Parliament's elections in May next year, as they bid to discourage Euroscepticism following the UK's decision to leave the EU, and as populist parties form alliances to shake up the EU establishment ahead of the vote.

Far-right populists in Europe, however, were celebrating the Sweden Democrats’ rise.

Marine Le Pen, leader of France's far-right eurosceptic National Rally, said on Twitter: “Another bad night for the European Union in perspective. The democratic revolution in Europe is underway!”

The Social Democrats and its allies have tried to appease anti-migrant sentiment in the country by urging more integration programs and resources for refugees in marginalized communities and to help them access education.

The centre-right Moderates had pledged to give more funds to the police and pushed for Swedish migration policy to fall in line with laws in other EU countries, such as Denmark and Germany. This would mean that migrants won't be able to stay unless they can prove that they can support themselves, enabling them to get a permanent residence permit.

They were also looking at ways of more quickly incorporating migrants into the workforce, such as promoting “simple jobs,” a reference to work that can be obtained with only an elementary education.