Demonstrators calling for nuclear disarmament staged massive protest marches in Paris and Brussels today, adding momentum to a growing political constituency opposed to deploying improved U.S. nuclear weapons in Europe.

The peaceful marches followed larger protests yesterday in London and Rome and in Bonn two weeks ago. They again dramatized the difficulties the Reagan administration faces in its efforts to upgrade U.S. weaponry in Western Europe to balance Soviet forces in Eastern Europe strengthened by mobile SS20 nuclear missiles.

The turnout -- unofficially estimated at well above 50,000 in both capitals -- underlined the popular appeal of antinuclear arguments that U.S. and European strategists fear will have to be taken into account by Western Europe's governments.

Such popular pressures already have resulted in NATO defense ministers endorsing the "zero option," as a possible negotiating position with the Soviet Union. West Germany, Italy and the Netherlands now reportedly want to push for a stronger U.S. commitment to that option, under which U.S. Pershing II and cruise missiles would not be deployed in Europe if the Soviet Union would dismantle its SS20 missiles.

Defense Secretary Caspar W. Weinberger, leaving London after a week of talks with NATO defense ministers, said such demonstrators should be taken seriously, but that they are not seeking their goal of peace in the best way, The Associated Press reported.

["It doesn't change the policies of the government," Weinberger told reporters at Heathrow Airport. "It is completely understandable. But it's the wrong way to get the result we all want -- no war."]

The French Commmunist Party and its Belgian counterpart played key roles in organizing and publicizing the protests. But marchers in both cities called for an end to deployment of Soviet SS20 missiles as well as a ban on the American missiles.

The antinuclear arguments arose here with increased volume last week following remarks by President Reagan admitting the possibility of nuclear exchanges limited to Europe. Seized on by the Soviet Union and widely commented on in European newspapers and television broadcasts, Reagan's comment also provided ammunition to orators at today's Paris demonstration.

"The recent declaration of Reagan on a nuclear war limited to Europe expresses out loud what military strategists have been planning for a long time," said a statement from the Communist Party-run French labor union, Confederation Generale du Travail, read to loud cheers at the rally.

The strongest fire in Paris and, according to news agency reports, in Brussels was directed against the United States. This seemed to reflect leftist tendencies of many marchers and protest organizers.

U.S. officials argue that it is unfair to protest NATO nuclear plans in Western Europe when the Soviet Union can deploy its weaponry without fear of popular demonstrations. As if to underscore the argument, the East German news agency ADN reported today that 50,000 protesters marched in East Germany to complain about NATO weapons but mentioned nothing about Soviet SS20s in Eastern Europe.

Lawrence Eagleburger, U.S. undersecretary of state for European affairs, complained here yesterday that Western Europeans tend to ignore this in their assessment of East-West relations, describing what he said is a "general tendency in the West to give the Soviets the benefit of the doubt while American initiatives are considered with suspicion and hostility."

His comments came in a statement submitted to an independent disarmament committee meeting here this weekend under the chairmanship of former prime minister Olof Palme of Sweden and with the participation of former secretary of state Cyrus Vance.

The U.S. official stand was endorsed today by French Defense Minister Charles Hernu, demonstrating France's strong endorsement of the Reagan administration plans to strengthen NATO's nuclear might while at the same time negotiating for mutual arms reduction with the Soviet Union.

France's Socialist government under President Francois Mitterrand has emerged as Europe's strongest backer of the Reagan administration plans for nuclear rearmament.

Despite France's traditional nuclear autonomy outside the NATO command structure, Mitterrand has expressed strong agreement with Reagan on the need to meet Soviet nuclear power in Europe. In addition, his government has gone ahead with French experiments on neutron weapons started under his predecessor, Valery Giscard d'Estaing.

Although only French weapons are stationed in France, U.S. neutron warhead production was a favorite target of demonstrators at today's Paris protest. Nothing was heard about the French neutron weapons program or France's own arsenal of nuclear warheads.

The French public in general has voiced little opposition to Mitterrand's pro-American defense policies or his pursuit of neutron weapons research. Although today's demonstration drew French men and women from a variety of leftist groups, its organization and promotion came largely from the French Communist Party.

Among groups represented in the march were the French Communist Party and its youth wing; an ecologically oriented group, Greenpeace; Women for Peace; and a variety of youth organizations to the left of the Communists.

Overall sponsorship was attributed to the World Peace Movement, whose stands often are aligned with Soviet foreign policy goals. But many of the youths and young adults who turned out in a chill autumn rain belonged to no group.