French conservatives on Sunday are choosing their nominee for next year's vital presidential election from among two former prime ministers with deep experience in government and differing views on how to prevent more terror attacks on French soil.

The contenders in the conservative's primary runoff — Francois Fillon, 62, and 71-year-old Alain Juppe — are both high-profile leaders of the center-right Republicans party. Fillon, who wants to focus on fighting Islamic extremism, is judged by many to be the front-runner.

The winner of the runoff ultimately could end up facing far-right National Front leader Marine Le Pen, who is banking on anti-immigrant, anti-Muslim and anti-establishment sentiments to sweep her to power in the general election set for April, with a runoff the following month if neither side wins a majority.

The incumbent, Socialist President Francois Hollande is expected to announce in the coming weeks whether he will seek re-election. The position of the French left has been weakened by Hollande's extreme unpopularity.

Fillon has enjoyed a strong boost in popularity in recent weeks. He promotes traditional family values and said he plans to reduce immigration to France "to a minimum."

Juppe is advocating a more peaceful vision of French society, based on respect for religious freedom and ethnic diversity.

The two also have strongly different views on how to deal with Russian President Vladimir Putin, with Fillon in favor of forging closer ties. He wants to drop sanctions against Russia over its aggressive actions in Ukraine and partner with Russia in the fight against Islamic State extremists.

Fillon insists "Russia poses no threat" to the West, while Juppe wants France to continue putting pressure on Putin on various fronts.

They both pledge to cut public spending, reduce the number of civil servants, raise the retirement age from 62 to 65, extend the work week beyond 35 hours and cut business taxes.

Fillon was the prime minister from 2007 to 2012 under President Nicolas Sarkozy, who was eliminated in the primary's first round a week ago and now is backing Fillon. Juppe was prime minister from 1995 to 1997 under President Jacques Chirac.

In the first round of primary voting on Nov. 20, Fillon won 44.1 percent of the votes, Juppe 28.6 percent and Sarkozy 20.7. A second round is needed because no candidate secured a majority.

All French citizens over 18 — whether they are members of the Republicans party or not — can vote in the primary if they pay 2 euros in fees and sign a pledge stating they "share the republican values of the right and the center."

They can vote in 10,228 polling stations open from 8 a.m. to 7 p.m. across the country.

More than 4.2 million people voted in the first round, which organizers considered as a significant turnout.

Results are expected Sunday night.