BELGRADE, Yugoslavia (AP) _ The government said Sunday it sent a special police unit to Kosovo because mounting ethnic unrest in the poor province bordering Albania could endanger national security.

The move, reported by the official news agency Tanjug, follows renewed demonstrations by thousands of people in Kosovo and means federal authorities have usurped the power of the province's police and judicial organs.

The southern province has been plagued by tension between its ethnic Albanian majority and the Serbian and Montenegrin minorities that comprise 15 percent of the population of the underdeveloped region.

A government statement did not say how many men are in the special Interior Ministry unit or when it was sent to Kosovo.

Observers in Belgrade said the force likely consists of a paramilitary unit of 4,000 to 5,000 men, including riot police, ''Swat teams'' and a variety armored vehicles.

Last weekend, several thousand Serbian and Montenegrin women demonstrated for three straight days in Kosovo to protest remarks attributed to a former ranking official that prostitution of Serbian women could halt allegedly frequent rapes in Kosovo.

The official, Fadilj Hodja, is an ethnic Albanian who retains great influence in Kosovo. This week, he was ousted from the Communist party for his alleged support of Albanian nationalism.

Marija Gasi, a spokeswoman for Kosovo's provincial information secretariat, told The Associated Press the police unit was ''probably sent because of recent demonstrations organized by Serbian and Montenegrin women.''

She was contacted by telephone at her home in Pristina, the Kosovan capital.

Tanjug said the introduction of ''extraordinary measures'' in Kosovo had been ordered by Yugoslavia's collective state presidency, made up of representatives from each of the nation's six republics and two autonomous provinces.

The presidency's statement, as reported by Tanjug, said ''danger exists for the further worsening of the situation in Kosovo, which could seriously endanger the security of Yugoslavia.''

Similar measures were introduced in 1981 when thousands of ethnic Albanians rioted in Kosovo to demand the province be elevated to a full-fledged republic and put on equal consitutional footing with the six existing republics.

Some Albanian residents of the province seek more autonomy or even unity with Albania. Under the 1974 Yugoslav constitution, Kosovo is administratively part of the republic of Serbia, but has some autonomy.

More than 22,000 Serbs and Montenegrins have left the province since the 1981 riots, in which at least nine people were killed and hundreds injured. The migrating minorities alleged harassment by ethnic Albanians.

Ethnic Albanians, in turn, say they are harassed by police and fined or jailed for acts such as singing Albanian national songs.

Thousands of Serbs and Montenegrins have organized public demonstrations in the province in recent weeks asserting they are being forced from their homes by Albanian nationalists.

Although the Serbian government has nominal jurisdiction over Kosovo, the province has been effectively autonomous with local police units in charge of law enforcement.

During the past two years, however, leading Serbian officials have repeatedly called for a curtailment of Kosovo's autonomy.

Sentiment about Kosovo has been running high in the Serbian and federal capital of Belgrade in recent months. Serbs frequently speak passionately about Kosovo, which is the historic cradle of the Serbian state.

Its population is 1.9 million, with the number of Serbs and Montenegrins in continual decline from some 500,000 in 1965 to about 200,000 at present. The region's ethnic Albanian population has Europe's highest birthrate.