Sweden's left- and right-wing blocs were neck-and-neck in an election on Sunday that saw support surge for the nationalist Sweden Democrats, exit polls indicated, as one of Europe's most liberal states turns right amid fears over immigration.

The ruling centre-left Social Democrats and Greens and their Left Party parliamentary allies were seen winning 39.4 percent of the vote, while the opposition centre-right Alliance were seen at 39.6 percent.

The Sweden Democrats, a party with white supremacist roots, rose to 19.2 percent from 12.9 percent in the previous election, the poll by public service broadcaster SVT suggested, depriving either of the mainstream blocs of a parliamentary majority.

Another, smaller poll by TV4 had the Sweden Democrats taking 16.3 percent.

A partial tally of the vote by the Election Authority is due between 2000 and 2100 GMT.

The process of forming a stable government out of the deadlocked parliament could take weeks and, potentially, end in failure.

Voters mark their candidates at a polling station in Stockholm. Photo: Reuters

Acrimony between the two main political blocs has defined Swedish politics for decades, and the rise of the Sweden Democrats - long a pariah grouping in parliament - has hugely complicated the political landscape.

The Sweden Democrats' success follows a surge in popularity for other far-right parties in Europe amid growing anxiety over national identity and the effects of globalisation and fears over immigration following armed conflict in the Middle East and North Africa.

The election will add to concerns in Brussels as the European Union enters campaign mode ahead of the European Parliament election in May, which could give more voice to eurosceptic groups and thwart efforts at closer EU integration.