PARIS (AP) _ France's Socialist Party, already weakened by election debacles, is suffering anguish over a book detailing President Francois Mitterrand's ties to Nazi collaborators during World War II.

Most of the party's old guard, particularly those close to Mitterrand, have avoided substantive comment on ''Une Jeunesse Francaise'' (A French Youth), written by investigative reporter Pierre Pean and published last week.

But younger party members have criticized the Socialist hierarchy for silence.

And they have voiced dismay at the book's revelation that Mitterrand had a close friendship through 1986 with Rene Bousquet, a senior police official in the pro-Nazi Vichy regime who oversaw the deportation of French Jews to concentration camps.

Bousquet was killed by a gunman last year while awaiting trial for crimes against humanity.

Pean's book, focusing on a period skimmed over in other biographies of the president, depicts a young man from a conservative, nationalist family who became an ardent follower of collaborationist leader Philippe Petain.

At 26, Mitterrand received the francisque, Vichy's highest award, from Petain. And when he joined the Resistance in 1943 - not earlier as he long maintained - he did so without repudiating his Vichy past, its ideology or his friends, Pean contends.

In an interview appearing in the Le Figaro daily Thursday, Mitterrand commented on the controversy for the first time.

''I never flirted with the extreme right,'' he was quoted as saying. ''I was a product of my milieu - the very classic French bourgeoisie, Catholic and traditionalist, and thus conservative. And patriotic... There was never a trace of anti-Semitism in my family, or in me.''

The Socialist Party's chief spokesman, Jean Glavany, told reporters Tuesday that Mitterrand deserved credit for candor.

''I denounce the politicians who would judge a man on a fraction of his life,'' Glavany said. ''Those who pass judgment today should remember that 100 percent of French people were supporters of Petain (a World War I hero) in 1940.''

Mitterrand, 77, is ailing from prostate cancer and will step down when his second seven-year term expires next May. His successor is likely to be the nominee of the conservative coalition that routed the Socialists in parliamentary elections last year.