Mr Persson (left) faces stiff competition from Fredrik Reinfeldt
Swedes have voted in what looks like being the closest election for many years.
The Social Democrats have governed Sweden for all but 10 of the 89 years since the country introduced parliamentary government. But they could lose power to the resurgent centre-right.
The final outcome may still be decided by voter support for a number of single-issue parties.
Who's in power?
The Social Democrat Party, led by 57-year-old Prime Minister Goran Persson, got the most votes in the last elections in September 2002. It formed a minority government and has relied on the backing of the Environment Party (Greens) and the Party of the Left to pass legislation.
Have the past four years been successful?
Despite overseeing strong economic growth, the administration has not had a smooth ride. Voters decisively rejected the euro in a referendum in September 2003, despite the government campaigning for Sweden to adopt the European single currency.
Foreign Minister Anna Lindh, widely seen as a likely successor to Mr Persson, was murdered while shopping in Stockholm the same month. Her death shocked the country.
The government was damaged by its slow response to the Asian tsunami in December 2004, in which 543 Swedes died. Even Social Democrat MPs on the Constitutional Affairs Committee joined in harsh criticism.
The Social Democrats have also been involved in a number of domestic scandals, including a case of a libellous e-mail sent from the Social Democrats' HQ and a membership subscription scandal in the party's youth organisation.
Who are the challengers?
Sweden's four centre-right parties - the Moderate Unity Party, the Liberals, Centre Party and Christian Democrats - are united behind one prime ministerial candidate, Fredrik Reinfeldt. Aged 41, he is leader of the Moderates, the largest of the four parties.
Extreme right-wing parties are standing for parliament in every Swedish county for the first time.
The June List, formed in February 2004 after Sweden voted against adopting the euro, is also standing.
Another newcomer is Feminist Initiative, which was formed by the ex-leader of the Party of the Left, Gudrun Schyman, in April 2005.
What are the opinion polls saying?
Nationwide polls conducted in June by the polling organisation Synovate Temo suggested an almost dead heat, with the present opposition centre-right leading the governing centre-left by 47.6 to 47.2%.
This lead increased to 50.1% against 47.2% in August. But polls conducted during the holiday season are notoriously unreliable and a centre-right victory is by no means assured.
A poll published on 5 September suggested the opposition was leading by two percentage points (48.9% to 46.9%) - too small a difference to be statistically certain.
None of the single-issue parties looks set to pass the 4% threshold for representation in the new parliament.
What about the Swedish economy?
Mr Persson can point to a flourishing economy and generous social welfare. Officially, unemployment in Sweden is around 6% and the premier has said strong economic growth will bring this down. By comparison, France has 9% and Germany 10.6%.
The Confederation of Swedish Industry however has criticised the cradle-to-grave welfare state, and says generous benefits often make it nearly as lucrative to stay at home as to work.
Are there any international implications?
Neither the centre-right nor the centre-left seems likely to push for another referendum on the euro.
Mr Reinfeldt would like to see Sweden join Nato "as Sweden already takes part in many Nato operations", while the Social Democrats wish to maintain Sweden's neutrality.
What has the campaign been like?
Both sides have suffered blows. Lars Danielsson, one of Mr Persson's top aides, resigned on 31 August over the government's tsunami response.
The opposition Liberal Party Secretary Johan Jakobsson quit in early September, regretting he had not "acted more forcefully" to stop a party worker hacking into the Social Democrats' computer network.
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