PRISTINA, YUGOSLAVIA, JULY 6 -- Yugoslavia's federal presidency acted today to head off the country's gravest constitutional crisis in a decade by demanding the repeal of a declaration of sovereignty issued last Monday by legislators in the western republic of Slovenia.

The presidency -- a collective, eight-member body that acts as head of state -- also denounced as unconstitutional a similar declaration of independence by ethnic Albanian deputies in the southern province of Kosovo.

The presidency has struggled to hold together Yugoslavia's quarrelsome nationalities since the death in 1980 of the country's post-war leader, Marshal Tito.

The presidency said the Slovenian declaration "violated the common interests of Yugoslavia's different nations," and it ordered the federal government to block implementation of the declaration.

The Slovenian declaration, passed overwhelmingly by the three-chamber parliament in Ljubljana, declared Slovenia a sovereign state with defense and foreign policies independent from Belgrade.

Of Yugoslavia's six republics, Slovenia and its neighbor, Croatia, are the only ones with democratically elected governments, following the defeat of the ruling Communists in multi-party elections this spring.

Slovenia's declaration was the strongest challenge yet to Belgrade's dwindling authority.

Political analysts in Belgrade say, however, that the most serious threat to Yugoslavia's stability comes not from Slovenia but from Kosovo, a province within the Serbian republic.

An unofficial declaration of independence by ethnic Albanian deputies in Kosovo's parliament has provoked a crackdown here in Kosovo's capital by Communist authorities.

Over 100 ethnic Albanian deputies issued their declaration of independence from Serbia, Yugoslavia's largest republic, on the steps of the parliament here after being locked out of the main chamber. As Yugoslavia's third-largest ethnic group, the ethnic Albanians demanded independence from Serbia and the status of a republic.

Serbia reacted Thursday by dissolving the Kosovo government and parliament and by silencing the province's Albanian-language television and radio broadcasts, which the Serbian media denounced as mouthpieces for "separatists" seeking to merge Kosovo with neighboring Albania.

Armed riot police stormed the TV studios and led away several staff members. Over 300 television and radio staff went on strike today to protest the police action and the dismissal of senior managers.

Police cradling AK-47 automatic rifles stood guard overnight at the TV station and the office of the main Albanian-language newspaper, Rilindja. "We're under occupation," said a journalist standing outside the newspaper office.

The province remained under an uneasy calm, with ethnic Albanians heeding appeals from opposition leaders not to take to the streets in protest.

In demonstrations last January, more than 30 ethnic Albanians were reported killed by Serbian riot police.

Serbian Deputy Prime Minister Momo Trajkovic defended his government's actions as necessary to counter an Albanian separatist threat.

At a news conference here, he said the Kosovo parliament could be reconvened when the situation improved. He warned, however, that Serbia would take "any measures" to protect its territorial integrity.

Ethnic Albanian deputies have appealed for help from federal authorities in Belgrade. But this year's head of the presidency is a Serb, Borisav Jovic, regarded as an old-style Communist and a colleague of Serbia's Communist president, Slobadan Milosevic. Milosevic is on record as declaring that Serbia will "never in a thousand years" give up Kosovo.