The Sweden Democrats have promised to sink any government that refuses to give them a say in policy, particularly on immigration.
In one of Europe's most liberal nations, it was the latest in a series of spectacular gains for far-right parties across the continent. But thanks to the Sweden Democrats, it became a question that couldn't go unanswered, Johan Hassel, the Social Democrats' worldwide secretary, said after the vote.
A senior German official is describing the Sweden's election, in which the governing party lost ground and a far-right party made gains, as a "turning point" for Sweden and Europe.
Sweden is often held up as the paradigm of socialist democracies, and rightly so.
The opposition Moderate party is also unable to take over because even an established alliance with the centre-right parties doesn't make up the numbers needed for a majority.
"In some sense we're happy the Sweden Democrats didn't grow more than they did", Liberal Party lawmaker Allan Widman told Reuters.
"We need a cross-bloc cooperation", he told party supporters on Sunday evening.
Overseas votes, which could make a slight change to the outcome, have not yet been counted and the final result is not expected until Wednesday.
The governing party is still likely to be either the first-place center-left Social Democrats or the second-place center-right Moderate Party, the.
With neither main bloc able to command a majority, the Sweden Democrats - who want the country to leave the European Union and put a freeze on immigration - could play a decisive role in negotiations over forming a government.More news: Cryptocurrencies continue to tumble on Goldman reports
The Sweden Democrats have been championing closed borders for years and want to severely restrict immigration and integration. Sweden now looks set for some complicated talks about which side can form a workable goverment.
With a steady rise in popularity of the Sweden Democrats, immigration has become the hot topic of the election.
"The Alliance will not govern or discuss how to form a government with the Sweden Democrats", he said.
STOCKHOLM - Sweden faced weeks of uncertainty after its mainstream center-left and center-right blocs emerged in a dead heat from an election on Sunday, while the far-right - which neither wants to deal with - made gains on a hardline anti-immigrant platform. The government's finance minister suggested refugees seek another country in which to claim asylum, while Prime Minister Stefan Löfven announced that the country would crack down on criminals, and the party declared that emergency border security laws from the height of the refugee crisis would be kept in place indefinitely. Both have built alliances with other smaller parties and while neither of these alliances have a majority (both stand at 40 percent), it might be - depending on how much those not part of the relative alliance object to it - enough for one of them. Other than that, little is known about the views of the party .
About 7.5 million voters were eligible to choose the next members of the 349-seat Riksdag, or parliament.
But Kristersson and Christian Democrats leader Ebba Busch Thor rejected his invitation, according to Dagens Nyheter.
Voting in the capital Stockholm after an "unpleasant" campaign, student Katze Collmar, 32, warned: "It feels like Sweden could take a step in this election that we won't be able to recover from very easily".
But it's also interesting to note that immigration may not have turned out to be quite such an important factor to voters than expected.
Responding to the call, Mr Lofven said he would continue to "calmly work" in his role over the next two weeks (when parliament opens), but acknowledged the election "should be the funeral for bloc politics".
Sweden's ruling party was headed for its worst showing in decades as voters flocked to an anti-immigrant party with white supremacist roots that was poised to become the third-biggest force in parliament.