From the declaration of a national day of mourning to nationwide calls for relief, the Soviet government is using the disastrous earthquake in Armenia to unify this country of disparate peoples, languages and cultures.

For nearly 50 years, World War II has served that purpose, and the government propaganda machine still pours out stories, documentaries and films recalling the Great Patriotic War, in which 20 million Soviets died and the Soviet Union suffered and triumphed as a nation.The space program, too, is portrayed as a national endeavor.

Today, the rallying cry is last Wednesday's earthquake, which by official estimate has killed more than 50,000 people. The government has moved with extraordinary speed to mobilize its vast media resources to inform and to draw its people together.

Official reports say the disaster has united Armenians and Azerbaijanis, whose ancient ethnic feud was rekindled recently when a territorial dispute resulted in violence in several cities, leaving about 60 people dead in the months before the earthquake.

But Monday, Armenian activists reported clashes with troops in the Armenian capital of Yerevan on Sunday over the arrest of several nationalist leaders. They said several people were wounded on both sides.

And some Armenian activists already have complained that authorities are shipping children orphaned by the quake out of the republic to be cared for. They have charged this would constitute "Russification," a derogatory term for the Russian majority's blurring of ethnic lines.

On Sunday, President Mikhail S. Gorba-chev angrily denied that orphans were being sent out of Armenia and said the activists were trying to create instability.

The Soviet Union is a huge country bound more by central rule than common purpose. Its people, spread over a sixth of the Earth's land, represent more than 100 nationalities, speak dozens of languages and in some cases feel more kinship with people from other countries than with their own.

Unlike in the United States, where a nation of immigrants prides itself on having formed a great - if imperfect - melting pot, centuries-old ethnic groups in the Soviet Union are struggling to maintain their separate identities.

The nation's 15 republics are held together by Kremlin power and a national armed force, but a citizen is more likely to refer to his or her nationality than call himself or herself a Soviet.

Gorbachev, who returned abruptly from the United States to rush to the stricken region, has made the earthquake a national cause. His personal leadership in a time of crisis has contributed to a sense of unity.

The state-run media, which once ignored natural disasters, have poured out information, telling and showing people throughout the country the extent of the damage - and the need. Newspapers and television have been just as quick to report the response.

The official Tass news agency has noted, for instance, that Russia, the Ukraine and Kazakhstan sent cranes to help raise the rubble, that a planeload of medical supplies was dispatched from a factory in Western Siberia and that tents and food were dispatched from Tadzhikistan.

There have been similar reports in the media daily.

The country, long isolated by suspicion and fear of foreigners, also has opened its borders to accept aid it once would have been too proud - or perhaps too self-conscious - to accept from outsiders.

Not only has assistance come from Moscow's socialist allies but from great and tiny nations around the world. The United States, Denmark, Algeria, Iran are among those mentioned in a single report.

Even assistance from nations such as Israel, with whom the Soviet Union has no diplomatic relations and intense political disputes, has been welcomed.

Gorbachev, who describes himself as a realist, has been pragmatic enough to break down barriers that his predecessors hid behind.

"Today, the preservation of any kind of `closed' societies is hardly possible," he told the United Nations the day the quake struck. "The world economy is becoming a single organism, and no state, whatever its social system or economic status, can develop normally outside it."