Preliminary results showed Sunday night that the opposition Social Democratic Party gained more than 31 percent of the support, the highest among the competing parties, Xinhua reported.
And, together with the Green Party and the Left Party, the alliance garnered 43.5 percent of the votes, while the centre-right government claimed 39.5 percent.
Meanwhile, the anti-immigration far-right Sweden Democrats party gained an unexpected 13 percent of the votes.
Due to the Sweden Democrats' huge gain, more than double what it achieved four years ago, the party has become a "balance of power" between the centre-left and centre-right blocs.
The "red-green" centre-left alliance appeared unable to form a majority in parliament and shall probably have to ally with smaller parties within the ruling centre-right bloc to form a new government.
Amid disappointing results, Fredrik Reinfeldt announced to resign as both the prime minister and the leader of his Moderate Party while addressing the Moderate Party Sunday night.
Meanwhile, Social Democrat leader Stefan Lofven is ready to lead the opposition "red-green" alliance as the next Swedish prime minister in the new government.
"All parties have a responsibility now. I'll talk to other parties. My hand is outstretched. I'll talk to the Greens, but also to other parties," said Lofven, in hopes of forming a stable majority government soon.
The final election results are expected to come out Sep 20 and the newly elected parliament will convene Sep 29, according to information posted at the website of the Swedish parliament.