Finnish politics could see a dramatic shift to the far right in elections on Sunday as an anti-immigration party aims to replace "rock star" Prime Minister Sanna Marin's Social Democrats.

The mother-of-one, who took office in 2019 as the world's youngest prime minister at age 34, is Finland's most popular prime minister this century, polls show.

But the latest polls on Thursday put her centre-left Social Democratic Party (SDP) in third place, behind the anti-immigration and nationalist Finns Party and the centre-right National Coalition, which held on to a thin lead.

"It is a very exciting situation and it's hard to say at the moment which party will be the biggest on election day," Tuomo Turja of the polling firm Taloustutkimus told AFP.

A top spot for the far-right Finns Party, and a far-right prime minister, would be a first in Finland -- though the party has previously served in government. It currently looks set to top its record 19 percent from the 2011 election.

Traditionally, the biggest of the eight main parties in parliament claims the top post and tries to build a government.

Marin leads a centre-left coalition of Social Democrats, the Centre, the Greens, the Left Alliance and the Swedish People's Party of Finland.

While some view her as a strong leader who skilfully navigated the Covid-19 pandemic and the country's NATO membership process, others say her partying scandals and youthful behaviour make her unfit for office.

"Sanna Marin is a polarising character. She has fans like a rock star, but on the other hand, she has a lot of people who can't stand her," Marko Junkkari, a journalist at daily Helsingin Sanomat, told AFP.

- Populist surge -
The leader of the opposition conservative National Coalition, Petteri Orpo, has focused his campaign on the economy, accusing the government of irresponsibly increasing public debt.

"The outlook is very bad. Our public finances will plummet and this will lead to the erosion of the foundations of our welfare society," Orpo told AFP.

Finland's debt-to-GDP ratio has risen from 64 percent in 2019 to 73 percent, which the National Coalition wants to address by cutting spending by six billion euros ($6.5 billion).

Marin has defended her track record and accused the National Coalition of wanting to "take from the poor to give to the rich."

Support for the populist Finns Party has surged since last summer, spurred by rising costs of energy and other goods in the wake of Russia's invasion of Ukraine.

The eurosceptic party wants a hard line on immigration, pointing to neighbouring Sweden's problems with gang violence and laying the blame on its large influx of migrants.

"We do not want to go the way of Sweden. We are highlighting the effects of a harmful immigration policy," Finns Party leader Riikka Purra told AFP.

While the party served in a centre-right government in 2015, it later split into two factions, one hard-line and the other moderate.

Only the hard-liners, which became the second-biggest party in the 2019 election, now remain in parliament.

The Finns Party sees an EU exit as its long-term goal and wants to postpone Finland's target of carbon neutrality for 2035.

- Shift to the right -
Negotiations to build a government are expected to be thorny.

The former heavyweight in Finnish politics, the Centre Party, has plummeted from the largest party in 2015 to record-low support, after sitting in consecutive right- and left-wing governments for nearly eight years.

It does not want to continue in Marin's current coalition, clashing in particular with the Greens.

Without the Centre's support, both the SDP and the National Coalition will have a hard time building a majority.

And Marin has ruled out forming a government with what she calls the "openly racist" Finns Party.

Orpo has said he will keep his options open, which gives him a central role in forming the next government, as both the Finns Party and the SDP would likely need him to obtain a majority.

"At the moment, the most likely scenario is a blue-red government based on the National Coalition and the SDP," Turja said.

While a left-right government is reasonably common in Finland's consensus-oriented politics, their opposing economic policies could complicate the task.

Another option would be a right-wing government with the National Coalition and the Finns Party.

While Orpo said the two "have their differences" when it comes to the EU, immigration and climate goals, "there are many things that unite us," such as economic policy.