A powerful bomb placed under a car outside the city's most fashionable synagogue killed four persons and wounded a dozen others shortly before the end of today's evening prayers. Seven of the injured were listed as being in serious condition.

The assault followed machine-gunning of Jewish landmarks in the city last week and dozens of anti-Semitic incidents throughout France this year. Jews in turn have denounced police countermeasures as inadequate and in some cases have called for Jews to set up their own security forces.

Leaders of the French Jewish community blamed the blast on a neo-Nazi international movement said to be responsible for the explosion that killed 13 persons at the Munich beer festival a week ago and the Bologna railroad station bombing in August that killed 83.

There were several hundred people in the synagogue when the bomb went off, destroying a number of cars and breaking windows almost a hundred yards down the narrow Rue Copernic in the chic 16th district of Paris.

Investigators estimated that 25 pounds of plastic explosive were used. Witnesses said flames went up to the fourth floor of surrounding buildings. A car was seen flying into the air, and the facades of five buildings were heavily damaged. Inside the synagogue, glass rained down on the congregation from a huge skylight, but injuries of all those in the building were described as minor.

Police said one of the four dead passersby was a 22-year-old motorcyclist who had approached the two police guards outside the synagogue to ask for directions while two other motorcyclists planted the bomb. The bombsetters reportedly left immediately, and the motorcyclist who had distracted the police -- whether or not intentionally -- was blown up as he drove off.

The body of another man ripped by the explosion could not be identified immediately. A third male victim was a 40-year-old neighbor passing by. The fourth was identified as a young Israeli woman who had gone to the service and apparently was leaving early.

A corner grocer in the next block said this was a much bigger explosion than when the German occupation forces blew up the same synagogue, the Reformed Jewish Union, in 1944.

Giselle Charzat, a Socialist member of the European Parliament who lives down the street, said she rushed out of her apartment and saw a number of people with blood gushing from their wounds and young Jews coming out of the synagogue singing, "Shalom, Shalom" -- peace, peace.

The explosion followed the machine-gunning on two successive nights last week of five Jewish community buildings and monuments in Paris.

Phone calls to news agencies attributed to the legal European Nationalist Fascists organization here claimed responsibility for those attacks and tonight's. But French Interior Minister Christian Bonnet dismissed the claims as meaningless. Sixteen members of the group were picked up by police after last week's attacks and released for lack of evidence.

Marc Frederiksen, the fascist group's leader, disclaimed the bombing and said he disapproves of such tactics.

The machine-gunnings, sparked a demonstration on Tuesday at the memorial near Notre Dame Cathedral for Jews deported from France during World War II. The memorial was one of the five targets hit last week.

Michael Williams, a French-speaking English rabbi officiating at tonight's service when the blast went off, said, "Deep down, I'm not surprised this happened. . . . This country doesn't take anti-Semitism seriously."

The French Jewish community has been torn by quarrels recently about whether to organize self-defense groups.A spontaneous demonstration of about 1,000 young Jews, chanting, "Fascism Will Not Pass, Israel Will Win," formed around the site of the blast and marched down the Champs Elysees to the Interior Ministry and the presidential palace, less than a mile from the explosion.

Interior Minister Bonnet received a delegation of Jewish community leaders last week expressing their concern that lack of police protection would lead to violence between young Jewish militants and right-wing extremists. Tonight he said:

"I want my police to find the perpetrators to avoid escalation between a community that is hurt because it feels threatened and small groups that act like cowards. . . . The interior minister is reacting as if he were himself a young Jew, but I beseech the young Jews not to provoke escalation."

Henri Hajdenberg, leader of a militant group called Jewish Renewal that has challenged the leadership of France's traditional Jewish establishment, arrived at the scene with a bodyguard in a black leather jacket. Hajdenberg announced a demonstration for Saturday and promised a march on the Interior Ministry to demand a purge of neo-Nazis in the French police.

He alleged that neo-Nazis have infiltrated into key positions to prevent the successful conclusion of investigations into anti-Semitic incidents. This summer a police inspector, Paul Durand, was dismissed after Italian investigations into the Bologna bombings showed he had been in frequent touch with violence-prone Italian neo-fascists.

French police list more than 150 anti-Semitic or racist incidents since 1977, 21 of them since July. This summer, 67 prominent Jews in southern France received written death threats signed by the National and European Action Federation, a group thought to have about 250 members.

Headed by Frederiksen, it was officially dissolved Sept. 3 by the Interior Ministry after protests by Jewish community organizations. Frederiksen them simply formed his new group.

There have already been clashes between Frederiksen's followers and a newly formed Jewish Defense Organization that claims to be training 300 young people to protect community buildings and disarm bombs.

Twelve Israeli detectives were sent here by Flatto Sharon, a member of the Israeli parliament protected by parliamentary immunity from extradition to France on charges of fraud. They arrived last week to organize a Jewish protection service. Representatives of Sharon recently said they intended to send 150 such men.

The Sharon group says it intends to get around French law barring such foreign parallel police by picking Israelis of French origin entitled to French passports and subcontracting to friendly French and Belgian detective agencies so they can legally carry guns.

The French and Belgian Jewish community leaderships have officially reacted in horror over Sharon's plan, which is also condemned by the Israeli government. But Sharon's men insist that the government of Menachem Begin is winking at their plan.

Leaders of the French Jewish establishment joined the interior minister in appeals for calm. But they were tinged with anger and warnings by such generally respected figures as Paris member of Parliament Jean-Pierre Pierre-Bloch, that French Jews "will give blow for blow it need be."

His father, another prominent leader, recalled tonight, "I said 48 hours ago, 'Gologna, Munich and soon Paris,' and I was called an alarmist. Well, tonight we have the proof that I was right."

He conceded that the French neo-Nazis are few in number. But, he said, what makes them dangerous is that they are part of a pan-European movement that gives them support from West Germany, Italy, Spain and Belgium.