A win for the opposition would halt a slide to the right in the past four years that has transformed Sweden from one of Europe's most welcoming countries for immigrants to one with among the toughest asylum regimes.
Left-leaning parties headed by Social Democrats leader Magdalena Andersson were projected to win 176 seats in the 349-seat parliament, while Prime Minister Ulf Kristersson's right-wing bloc was seen winning 173 seats, the election authority said.
"If this result remains valid, Sweden will get a new government," Andersson told supporters after the preliminary results were published on Sunday night.
The far-right Sweden Democrats, who hoped to join a government for the first time in the event of a win for the ruling bloc, lost an estimated 11 seats, projections showed.
Party leaders, however, have been cautious about claiming victory or accepting defeat with 353 election districts still to be counted and overseas votes not likely to be tallied until Wednesday.
In 2022, exit polls initially gave a narrow victory to the centre-left, which proved premature, and Kristersson showed no sign of conceding when he spoke after Sunday's election.
The blocs were separated by just 0.45 per cent of votes on Monday morning.
If a centre-left win is confirmed, forming a government could take some time.
While Social Democrat leader Magdalena Andersson will be prime minister, it remains unclear which other parties will be in her government.
The Centre Party has refused to share power with the Left Party, which says it must be a part of any coalition.
"It could very well be a complicated process to form a government," the speaker of the parliament, Andreas Norlen, told Swedish television.
The result of the election is unlikely to change the course of Sweden's pro-Western and pro-Ukraine stance and its effect on immigration and economic policy is unclear.
Andersson has promised voters more spending on welfare, a fairer tax system and a focus on social cohesion.
But while the Greens and Left parties want the rich to contribute more, the Centre Party is against a billionaires' tax.
The Social Democrats are unlikely to ease immigration rules, as campaigned for by the Left and Green parties, with Andersson saying Sweden has to continue its tough line.
with Reuters
