By Brian Love and Richard Balmforth

PARIS (Reuters) - Former prime minister Francois Fillon looked on Friday to be in a strong position to claim his center-right party's nomination to contest next year's French presidential election as he and rival Alain Juppe held final rallies of the primary campaign.

In an impassioned speech to supporters in Paris, Fillon, 62, struck a strong patriotic note, vowing to halt "the decline of France" under the ruling Socialists by sticking to what he said was a realistic program that included ending the 35-hour working week and making big savings by slashing public spending.

Juppe, 71, also a former prime minister, defended his more moderate policies, telling supporters: "I'm not going to engage in any grandstanding against our public service. I want to manage, not demonize, it."

A new survey issued on Friday night saw Fillon as clear favorite, winning Sunday's vote with 61 percent against Juppe's 39 percent.

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

Fillon, who defied predictions to emerge as surprise winner of Les Republicain' first-round primary on Nov. 20, 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.

Even as their supporters prepared for 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.

POLLS

Le Pen's National Front party, which is anti-immigrant and anti-European Union, 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 President Francois Hollande's socialists 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 credibility 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 rock-bottom and fell to just four percent in one recent survey.

"BRUTAL" POLICIES

Both Fillon and Juppe 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.

Juppe, speaking to a rally in the town of Nancy on Friday night, confirmed that intention, saying: "I'm not going to freeze the hiring of police, teachers etc during the next five years because that is impossible."

Fillon hit back at Juppe, who in Nancy, repeated charges that his rival was pursuing "brutal" social programs.

"What is brutal is the mass unemployment which is ruining millions of French people. What is brutal is the delinquency that is destroying entire districts. What may be brutal in the future is the day when the debt wall collapses on us," Fillon said.

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 militants working from the group's Middle East strongholds.

Another security scare was triggered by the killing of a woman at a home for aging nuns and retired Roman Catholic missionaries near Montpellier in southern France.

Police said later a 47-year-old former soldier had been arrested on suspicion of killing the woman, but that there were no links to Islamic militancy.

(Additional reporting by Bate Felix; Editing by Andrew Heavens)