London — The political center-right around the world is forming what it hopes will be a counterweight to the socialist left. The new body, to be called the International Democratic Union (IDU), will bring together center-right political parties in a score of countries to promote and defend:
* Individual freedom.
* A limited role for the state in private lives.
* A free-market economy, with some elements of the welfare state retained.
* An anti-Soviet, pro-NATO foreign policy.
The move reflects a growing self-assertiveness of the center-right following electoral victories by President Reagan, Chancellor Helmut Kohl, Prime Minister Margaret Thatcher, and others since 1979.
Signs of center-right strength have emerged in other countries as well. Recent municipal elections in France showed a trend from President Mitterrand's Socialists back toward the center. A center-right party is in coalition with the governing Socialists in Austria. Voter sentiment in both Iceland and Italy is veering to the right.
A recent count by the weekly Economist shows that only five of the 24 leading countries of the industrial world have genuine left-of-center governments. The magazine adds that two of the five (Spain and Australia) have ruling parties that are ''really more liberal than left.''
The IDU, which is to be launched here tomorrow, is clearly designed to oppose the Socialist International, which was founded in 1951.
The Socialist International speaks out frequently on world events and has served as a forum for former German Chancellor Willy Brandt, Sweden's Prime Minister Olof Palme, and former Austrian Chancellor Bruno Kreisky. It says it represents 15 million members and 80 million voters.
Mr. Reagan, Mrs. Thatcher, Mr. Kohl, and Bavarian CSU leader Franz Josef Strauss say it is time to organize their ideas to combat the Socialist International left. Why, they ask, should the Socialist International make pronouncements on world events while the center-right stays silent?
Mr. Reagan and Mrs. Thatcher see economic recession creating new interest in their ideas. Voters, they say, have reacted against the parties that were in power in the late 1970s and are looking for new leaders and ideas.
Another aim of the IDU is to help conservatives who have been defeated in recent times, such as those in Sweden, Spain, Portugal, and Greece. The object is to make available the experience and expertise of US Republicans, British Conservatives, the West German coalition of the Christian Democratic Union (CDU) and Christian Social Union (CSU), and others.
Mrs. Thatcher, fresh from her successful reelection campaign, has taken a strong interest in the new group. She will be playing host to a lunch Friday at 10 Downing Street for those coming to London for the afternoon founding ceremonies.
President Reagan is sending Vice-President George Bush to represent the Republican Party. Mr. Bush was scheduled to arrive here the day before as part of a broader visit to West Germany, Denmark, Norway, Sweden, Finland, the Irish Republic, and Iceland.
Organizers at the British Conservative Party headquarters here say that the US Democratic Party will also be represented, by Charles Manatt of the Democratic National Committee.
In London for the opening will be center and center-right prime ministers from Norway and Denmark, and conservatives from France (the mayor of Paris, Jacques Chirac, head of the Rassemblement Pour la Republique), Canada, Japan, Australia, New Zealand, Spain, Portugal, Cyprus, Sweden, Finland, Greece, Malta, and Austria. The IDU will have a secretariat in London and will hold its annual meetings here.
Why hasn't there been an international center-right organization before?
''It's an excellent question,'' said the IDU's new executive secretary, British Conservative Party official Scott Hamilton, in an interview. ''The Socialist International movement has been essentially international in outlook, whereas our parties have developed for national reasons. We have been slow to come together. . . .''
Mr. Hamilton is head of the British Conservative Party's international office at party headquarters on Smith Square in London. His chairman will be an Austrian, Alois Mock (the chairman of the Austrian People's Party). The treasurer is a Canadian, Allan Lawrence (a member of Parliament and official of the Progressive Conservative Party).
According to Mr. Hamilton, the idea for the IDU grew after the first elections to the European Parliament in the late '70s. Conservatives around Europe discovered that they had much in common and formed the European Democratic Union, based in Vienna. The EDU is also headed by Dr. Mock.
More impetus came with Mrs. Thatcher's first election victory in Britain in 1979. When Mr. Reagan came to power in the US, the Republicans asked for associate membership.
Last year the idea spread to the Pacific. Led by the US Republicans and the Japanese Liberal Democrats, a staunchly anti-communist Pacific Democratic Union was formed, with a secretariat in Canberra headed by Australian Liberal Party leader John Atwill.
Now the new IDU is to be an umbrella organization. Mr. Hamilton says it will include but not replace the European and Pacific groups.
''There has been some discussion,'' Mr. Hamilton said, ''of another regional democratic union linking North and South America. One needs to be cautious, however, given the current situation in Central America. . . .
''There's also interest in Africa and Asia, and in helping democratic center-right parties to grow stronger there. . . . As for the IDU itself, we see the main value at the moment as exchanging ideas and talking to each other.''
There will be difficulties ahead. Some parties are more to the center, some more to the right. Mr. Reagan and Mrs. Thatcher will want to speak out against the Soviet Union, but the Finnish Kansallinen Kokoomus Party will have to dissociate itself from any such statements, given Finland's complex ties with Moscow.
The IDU does hope to spotlight what it regards as dangerous tendencies within the Socialist International, such as the readiness of Socialist International to ''do business with'' far leftists on the fringes of the British Labour Party and in other countries. Another example of the nondemocratic far left cited here is the Jewel Party in Grenada.