In the greatest outward display of national unity in decades, more than 100,000 French men and women of all political and social categories marched through eastern Paris today to protest a bomb blast outside a synagogue that killed four persons and wounded a dozen others last Friday.

The demonstration, the largest such gathering since the end of World War II, coincided with fresh outbursts of violent anti-Semitism in several French cities and towns. A Jewish-owned grocery store in Grenoble and a bar in Marseilles were the targets of bombing attempts today, and dozens of Jewish stores or homes were attacked in Montpellier in southern France and several smaller towns. There were no reports of injuries.

The National Assembly voted unanimously to suspend its afternoon session so that legislators could participate in the march along the traditional route of leftist processions from the Place de la Nation to the Place de la Republique. All of France's major vital labor unions joined in the call by an antiracist organization for a two-hour strike to allow workers to take part in the march, which was scheduled for the last two hours of the normal work day.

In cold, gusty weather, hundreds of organizations of all kinds, including all of the country's feuding major political parties and most of the numerous minor ones sent delegations to express their outrage about the worst display of anti-Semitism in France since World War II.

The unity stretched, in a constantly repeated phrase, "from Marchais to Rothschild" -- meaning from Communist Party leader Georges Marchias to the conservative banking family that heads the French Jewish establishment.

Several factors appeared to explain why an incident in which far fewer people were killed than in the recent Bologna railway station or Munich beer festival bombings has rocked the French far more than the Italians or the West Germans.

Many people could be heard in private conversation saying that, after all, only four people died after last Friday's explosion in the Rue Copernic and wondering whether so much fuss would be made a Jewish institution had not been targeted.

The first factor is simple that the French, despite all the verbal violence of their political rhetoric, have developed a genuine horror of blood. It is a constantly repeated source of national pride that the nightly barricades battles during the May-June 1968 student revolt resulted in only one known death.

There has been frequent stress in France on the superiority of a political culture that, despite even deeper political divisions than its neighbors, has avoided the murderous terrorism of Italy, West Germany and Spain.

Another factor is that an attack on the Jews, even if it actually missed its maark and hit mostly non-Jewish passers-by, has revived national guilt complexes about the nonheroic behavior of most Frenchmen under the German occupation, when French Jews were being deported by the trainload. The U.S.-made Holocaust television series was only shown in France after a major campaign during which European Parliament President Simone Veil threatened to make a public scandal if it were not broadcast.

Jews have had a special relation with France ever since the French Revolution of 1789 made France the first country in modern times to make Jews equal citizens. Napoleon conquered Europe with those ideals in his baggage trains, breaking up the continent's ghettoes as he went. But France has never been a completely confortable place for the Jews because there was always a fraction of the population that never fully accepted the revolution.

In any event, today's outpouring of protest only barely covered continung deep differences of all kinds among the French. The forms of the reactions to the bombing outside the synagogue here also revealed the persistence of the social class and political divisions that have haunted France since the French Revolution.

Leaders of practically all the major political groups accused their rivals of trying to exploit the emotions of the moment to their own advantage. As one leader expressing a frequently heard theme said, "If I march in the procession behind groups with banners I disapprove of, there is no political significance."

The Socialist and Communist parties and the unions associated with them were the first to heed the call to march and were apparently at first surprised to find even this momentary reestablishment of their former unity of action.

The party of President Valery Giscard d'Estaing and the Gaullists hesitated for days before bowing to public pressure to join in. Gaullist Party leader Jacques Chirac finally said today that his party would be represented "to express its solidarity with the national elan against racism," but not to accept the ideas or hidden motives of other groups.

The most notable absentee was any official representation of the Giscard government. Simone Veil, a prominent former Cabinet minister under Giscard and a Jewish wartime deportee, said she regretted the "relative discretion" of the government.

Giscard and his ministers have confined themselves to expressing their indignation, but none of them has participated in the wave of demonstrations that has engulfed the country since the explosion Friday night.

One reason may be that a favorite theme of the demonstrators has been demands for the resignation of Interior Minister Christian Bonnet, who had publicly minimized the danger from neo-Nazi groups after four Jewish community buildings in Paris were machine-gunned without loss of life in the week before the Rue Copernic bombing. Except for statements -- also criticized -- that he made at the site soon after the explosion, he has been publicly silent.

Bonnet has made no reply to allegations by the two largest policemen's unions that he was given a list of 30 neo-Nazis who had infiltrated the police, often into high positions. The police union leaders who made the accusations were called in by police internal security investigators to give their evidence.

Delegations of the two unions were heavily applauded in today's march.But spokesmen for other, much smaller police unions accused them of discrediting the corps. The two unions allege that neo-fascists were welcomed into the police when its main political mission seemed to be the struggle against extreme leftist groups after the near-revolution of May-June 1968. The reasoning allegedly was that extreme rightists would be highly motivated to root out leftists.

The government may also have electoral reasons for not wanting to appear overly enthusiastic about this week's unusual solidarity with the French Jewish community. The government has been depicted, even by spokesmen for groups normally sympathetic to Giscard, as having at least some moral responsibility for the climate that has allowed the neo-Nazis to grow. This seems at first glance to have hurt the president's reelection prospects much more than widely publicized scandals such as his acceptance of diamonds from deposed Central African emperor Bokassa.

Yet it is a sociological fact that, regardless of this week's outrage, large numbers of Frenchmen are have become willing to tell pollsters, for instance, that they are at least social anti-Semites. Some of those voters are Communists. But the bulk of them have no mainstream party except the Giscardists for which they can comfortably vote, since the Gaullists and Socialists have deep roots in traditions that firmly reject anti-Semitism.

As it becomes increasingly apparent that the bulk even of conservatives in the 600,000-member Jewish community may be lost to Giscard in a race that now appears tighter than expected, his strategists may calculate that he cannot afford to offend the portion of the population for whom the French Revolutionary slogan of "Liberty, Equality, Fraternity" is simply cant.