The proclamation of martial law in the province of Kosovo has brought Yugoslavia face to face with the tension of the post-Tito era. Less than a year after his death, ethnic riots have threatened -- in the words of senior Yugoslav officials -- the country's independence and territorial integrity.

In an almost automatic reaction, Yugoslav authorities have blamed unnamed "foreign enemies" for the disturbances in the autonomous province, which is peopled predominantly by ethnic Albanians.

The major Yugoslav newspaper Politika took a somewhat different approach. It noted the conclusion by Kosvo communist leaders that "intenational interests" were involved in the riots in which nine persons were killed and more and 250 injured. But Politika said it was too early to make broad conclusions and called for an open discussion of "political and human" errors to restore the truste between various ethnic groups.

"It is easy to punish the guilty," the paper said, "but it is much important that trust gradually be restored."

Yugoslavia is a multinational country comprising six republics and two autonomous provinces. During his long rule, Tito was able to impose a degree of coherence on these regions that led to a broad national identity despite their disparate cultures.

Kosovo embodies the sharpest contradictions of Yugoslav community.

Roughly half the size of Maryland, it is a remote region of mountain ranges and fertile valleys where courtesy and hospitality coexist with out-bursts of cruel violence and deep ethnic and religious hostilities.

For Serbs, the largest Yugoslav ethnic group accounting for more than 9 million Yugoslavia's 22 million population, Kosovo has a special meaning. It was the site of a medieval Serbian state that was destroyed by the Ottoman Turks in 1389. For six centuries every Serbian child has been reared on legends and folk songs about the battle that took place on the field outside Kosovo's capital, Pristina.

What the Serbs regard as their ancestral province is now the home of more than 1 million ethnic Albanians, or 75 percent of Kosovo's population. The Serbs there are a minority whose proportion of the population is dwindling because of the exceptionally high birth rate of Kosovi Albanians, which stands at about 29 per 1,000, the highest in Europe.

The demographic trend has been intensified by a slow but steady exodus of Serbs from Kosovo after they lost control of the province and it was granted full home rule in 1968. Since then Albanians have come to dominate the political life of Kosovo. They have replaced Serbs in key positions, established an Albanian university in Pristina and made Albanian the dominant language in the province.

What makes ethnic and linguistic strains even more complex is the fact that the Serbs are Orthodox Christians while the Albanians are overwhelmingly Moslems. given a bloody history of religious feuds, it was perhaps inevitable that the nascent Albanian nationalism would eventually take on exclusivist overtones.

Thus far, ethnic disturbances in Kosovo, including the latest riots, reflected general dissatisfaction with the economy. Kosovo is the poorest of all Yugoslav provinces. Its per capita income is roughly one-sixth of that in the northern republic of Slovenia.

Other Yugoslav provinces have contributed substantial funds to assist Kosovo's economy and these contributions account for roughly 70 percent of the Kosovo budget. It relatively quick development has been remarkable, although the industries created have a poor record of efficiency. The rich agricultural potential has not been exploited fully.

But the combination of high fertility and the massive influx of students into Pristina University (it has 35,000 students) has creasted enormous disproportions in the province.

The issue of economic disparities, however, is not the critical long-term problem of Kosovo.

Rather, the root of the problem lies in the number of ethnic Albanians living in Yugoslavia and their location along the Yugoslav side of the border with Albania. In addition to more than 1 million Albanians in Kosovo, there are approximately 300,000 ethnic Albanians living in the neghboring regions of Montenegro and Macedonia. The total population of Albania is estimated at 2.2 million.

How long can these two large communities of the same people remain politically separated? The fact that Yugoslavia provides a much higher standard of living and greater political and individual freedom to its Albanian population than does the state of Albania itself has not precluded separatist tendencies among educated Kosovo Albanians.

There is little doubt taht students and discontented intellectuals spear-headed what seems to have been a full-blown nationalist protest. The explosive situation for Kosovo could arise when nationalist ambitions and economic strains coincide.

While the rioters advanced slogans calling for Kosovo to be upgraded from an autonomous province of Serbia into a full-fledged constituent republic, there were more nationalistic slogans calling for a "greater Albania" that would include Albanian minorities not only in Kosovo but also in Montenergo and Macedonia.

Such nationalist calls present a political problem for Belgrade since the thought of secession by what the Serbs regard as the cradle of their nation is enough to provoke atavistic feelings against Moslem "infidels." Even the granting of full republic status to Kosovo is politically difficult because it would symbolize the destruction of its historical and cultural links with Serbia proper.

So far, the long-term prospects for Kosovo have not been openly discussed either by Yugoslavia or Albania. But relations between the two countries have sharply deteriorated following the recent riots. Albani publicly defended the rioters and accused Yugoslavia of brutally quelling disturbances. Yugoslavia in turn charged that Albania's statements represent "a clear political platform of territorial claims" against Yugoslavia.

The acrimonious exchanges bring to the surface another contentious ethnic issue in the Balkans.

One factor that has been held Yugoslavia together in the past was the belief that any sign of internal weakness will be exploited by hostile neighbors and eventually by Moscow. This, in particular, was the case with Bulgaria's claim in Macedonia. It is unlikely, however, that Albania under its present leadership would turn to the Soviets for help. For more than two decades Albania has pursued Stalinist policies at home and isolationism abroad. For ideological reasons, it broke ties first with the Soviet Bloc and later with China.