OSLO - Voters shunned Norway's far-right and far-left parties and piled into the political centre in local elections six weeks after a far-right militant killed 77 people in attacks targeting the centre-left Labour Party, early results showed.

With most votes counted the centre-right Conservatives were surging some nine percent over their prior local election result, a performance analysts said could presage a challenge to the dominant Labour Party in 2013 parliamentary elections.

"The Conservatives are the big winners," said Haavard Narum, political columnist at newspaper Aftenposten, noting that the party was heading for its best result in city and county elections since 1979.

He said their advance to about 28 percent of votes nationwide came at the expense of the anti-immigrant Progress Party - the day's biggest loser.

Progress leader Siv Jensen blamed her party's poor showing in part on national revulsion over the July 22 bombing and shootings by Anders Behring Breivik, a former Progress member and self-proclaimed enemy of multiculturalism.

"It (the attack and aftermath) has been a special situation that put a damper on the campaign," Jensen told broadcaster NRK after partial returns showed her party with about 12 percent support, down seven percent since 2007.

While Labour, which has led Norway's three-party "red-green" government since 2005, was advancing more than two percent to about 32 percent, its Socialist Left coalition partner was plunging to its worst result since 1979, at about 4 percent.

Kristin Halvorsen, the Socialist Left leader since 1997, announced after the vote that she would step down in early 2012, a resignation Narum said could shake the political landscape.

"That is a very bad sign for the red-green coalition and I would not be surprised if the Socialist Left Party leaves the government next year," he said.

He said many in the Socialist Left think Labour has taken the partnership for granted while ignoring socialist environmental and foreign policy demands.

Prime Minister Jens Stoltenberg said he felt "secure" that his government would last through 2013.

Narum said the "sympathy effect" enjoyed by Labour after Breivik's attack - which saw its support exceeding 40 percent in some surveys in early August -- appeared to have largely disappeared by election day.

Monday's voting in city and county councils across Norway has no immediate effect on the Labour-led government's slim majority in the national parliament.

Analysts said Conservatives and their right-wing partners would control a larger number of Norway's 429 municipalities and 19 counties than before, but it was too soon to say how many ahead of local negotiations.