Former prime minister Francois Fillon looked to be in a strong position to claim his centre-right party's nomination to contest next year's French presidental election as he and his rival, Alain Juppe, headed into the final rallies of the primary campaign on Friday.

The ballot on Sunday will send one of the two veteran conversatives into an electoral battle that opinion polls say will boil down to a duel against far-right leader Marine Le Pen.

Fillon, who defied predictions to emerge as surprise winner of Les Republicains first-round primary last Sunday, is tipped to win the second round with two-thirds of the vote. He was also boosted by a convincing performance in a televised debate on Thursday.

Even as their supporters prepared to stage the last rallies, Paris's chief prosecutor issued a reminder of the security threat that has hung over France since Islamist militants killed 130 people in attacks in Paris in November 2015.

Le Pen's anti-immigrant, anti-European Union party has made big inroads against mainstream left- and right-wing parties as France struggles with a jobless rate of 10 percent and insipid economic growth. The Paris massacre and other militant attacks have also fueled support.

But with the ruling Socialist Party of President Francois Hollande in disarray and opinion polls showing a majority of voters opposed to seeing the far-right in power, many pollsters are seeing Fillon -- France's closest thing to a genuine conservative on both economic and social issues -- as having the best chance of becoming president.

Opinion polls have, however, lost credibilty since Donald Trump's surprise win in the U.S. presidential election and the Brexit vote to take Britain out of the European Union.

With the Socialists yet to declare a candidate and with independent candidates, including former Economy Minister Emmanuel Macron in the field, many pundits say that much could change between now and next May.

Polls for months have predicted that Le Pen would qualify for the second round of the presidential election but lose the run-off to a mainstream right-winger.

The unpopular Hollande is expected to say by the end of the first week of December if he will seek a second term, though his approval rating is at rockbotton and fell to just four percent in one recent survey.

Both Fillon, 62, and Juppe, 71, propose supply-side economic measures including cuts in public spending and raising the retirement age, although Fillon promises more drastic and faster measures.

While both men want to trim civil service headcount, Juppe, unlike Fillon, has promised to hire another 10,000 police officers to help improve law and order and combat the threat from militants.

Sunday's vote comes hard on the heels of yet more scares in a country which has been under emergency rule since the 2015 Islamist gun and bomb attacks.

After police swoops last weekend, public prosecutor Francois Molins said on Friday that five people under interrogation were suspected of having planned an assault in and around Paris on Dec. 1 under the direction of Islamic State (IS) militants working from the group's Middle East strongholds.

"A Strasbourg commando team, and also a man arrested in Marseille, were given instructions to acquire arms," Molins told reporters.

"The instructions were given by a commander from the Iraqi-Syrian region via encrypted (mobile phone) apps," he said.

His confirmation of suspicions that the Islamic States was behind the attack planning coincided with another scare triggered by the killing of a woman at a home for ageing nuns and retired Roman Catholic missionaries in southern France. The incident was later said to have no links with Islamic militancy.