By Ingrid Melander

PARIS (Reuters) - France's frontrunner for the conservative ticket in next spring's presidential election hit back on Tuesday after a personal attack from his rival, heralding a hard-fought final week of campaigning before the primary run-off this weekend.

Francois Fillon, who unexpectedly won the last vote by a wide margin against the then-favorite, Alain Juppe, were both known until now as mild-mannered former prime ministers.

Whoever wins the candidacy is expected to beat far-right National Front leader Marine Le Pen to the presidency.

On Monday, Juppe started the war of words by branding Fillon's conservative social views and reservations about abortion as "outdated" and said his free-market economic policies reflected "a great social brutality".

Speaking on Tuesday to lawmakers supporting him, Fillon retorted: "After calling me an ultra-liberal, I am now billed as a medieval reactionary. This is grotesque and ridiculous, let's focus on the real issues."

Later in the day, Juppe reacted to Fillon's remarks on Monday that Russia did not constitute a security threat, saying that France would have to engage in a dialog with Russia over issues such as Ukraine and Syria but that Paris would have to remain faithful to its principles.

"Talk to Russia doesn't mean becoming 'yes' men," Juppe, who has also been foreign minister and defense minister, said at a rally in Toulouse.

Juppe, 71, presents himself as a more modern centrist, able to win a majority of open-minded conservatives, centrists, undecided voters and even some left-wing voters. Former president Nicolas Sarkozy, who came in third in the first round, has thrown his support behind Fillon.

Anyone can vote in the primaries, which are not restricted to members of the conservative Les Republicains party.

CATHOLICS AND ABORTION

Born in a traditionally Catholic region of western France, Fillon, 62, is backed by groups that oppose France's same-sex marriage law. He says he wants to tweak that law to limit same-sex couples' adoption rights.

As a Catholic, he is personally opposed to abortion but has said he would not try to change the 1975 law that legalized it.

Juppe insisted on Monday that Fillon's statements on abortion were ambiguous and some of his positions on society represented a "backward step".

"Fillon belongs to a traditionalist family. I am more open to modernism," he told France 2 television.

In a Tuesday radio interview, he again urged Fillon to clarify his stance on abortion.

Later, Fillon told reporters: "I never thought my friend Alain Juppe could sink so low."

At a news conference at his headquarters, legislators in a "Women with Fillon" support group took to the stage to say how "shameful" and divisive they considered Juppe's comments to be.

"Panic and bitterness are bad counselors," said Valerie Boyer, campaign spokeswoman for Fillon. "We're shocked that such divisive societal questions are raised within our political family to divide us."

Another lawmaker, Isabelle Le Callennec, echoed Fillon's reservations by declining to say abortion was a fundamental right in France, as pro-choice activists assert. "It's allowed by law," she said. "One should not trivialize abortion."

Socialist Prime Minister Manuel Valls told lawmakers on Tuesday that Fillon's program was "reactionary on many societal issues".

(Additional reporting by Jean-Baptiste Vey and Matthias Blamont; Editing by Louise Ireland)