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At 31, Jimmie Aakesson has already spent five years at the helm of the far-right Sweden Democrats and is credited with a populist makeover that brought them a 20-seat windfall in weekend polls.

Out of nowhere his party has now become a power-broker after an inconclusive election which left the outgoing ruling centre-right Alliance scrambling for support in a hung parliament.

Aakesson pledged his party would act responsibly, in his first public comments after its surprising showing.

"We won't cause problems. We will take responsibility. That is my promise to the Swedish people," he told ecstatic supporters as near final results handed them 5.7 percent of the vote and their first seats in parliament.

"I am overwhelmed and it is hard to collect my thoughts. Today, we have written political history," he said.

Aakesson joined the Sweden Democrats at 15 and later helped to catapult the party from an obscure movement with a neo-Nazi past and virtually no voter support to a polished populist group.

When Aakesson joined the Sweden Democrats in 1995, "there were still members who showed up at meetings dressed in Nazi uniforms," political scientist Sofia Nerbrand said recently.

Aakesson, with his slight build, dark hair and glasses, has helped radically change Swedes' perception of the party.

He took over as leader in 2005 when the party had little popular support and the following year took nearly 3.0 percent in the general elections. Four years on, that support has nearly doubled, crucially passing the four-percent threshold for parliamentary representation.

With 20 seats in the new parliament, it could potentially determine the political make-up of Sweden's government after neither mainstream bloc secured a majority.

The Sweden Democrats emerged from the ultra-nationalist "Keep Sweden Swedish" movement, and Aakesson acknowledges it has counted neo-Nazis among its supporters.

"That's the old Sweden Democrats. Today we are different and voters see that," he told AFP before the election.

Born in 1979 in Soelvesborg, northern Sweden, Aakesson has held a seat on the local council since the age of 19. But he has never missed a chance to bring his party's main concerns to the fore, constantly evoking links between immigration and crime.

"Not all immigrants are criminals, of course, but there's a connection," he told AFP.

"We have a conservative point of view. Immigration and criminality policies are the most important and on that we differ from others," he said.

With his calm manner and choir-boy looks, Aakesson is the public face of his party, according to political scientist Anders Hellstroem.

The broader leadership consists of a "Gang of Four" -- Aakesson, Richard Jomshof, Bjoern Soeder and Mattias Karlsson -- all "leading the ideological development of the Sweden Democrats from extremism to populism," Hellstroem told AFP.

While the party is trying to distance itself from its extremist past, it is not afraid to be provocative, Hellstroem said.

"One could say that they try to push the limits of what is legitimate speech, balancing on the fringe of the acceptable," he said.

The Sweden Democrat's discourse has however until now been largely ignored by the mainstream media and political establishment.

But all that is about to change, as Aakesson reminded his troops on Sunday.

"We were exposed to censorship, we were exposed to a medieval boycott, they ... excluded us. We were denied advertising in many newspapers, we were in every possible way treated as something other than a political party," he charged.

"But despite all that, we scored a fantastic result," he shouted, stressing that the win would allow the party to "grow even more."