BELGRADE, OCT. 1 -- Yugoslavia slid toward civil war tonight as moves by riot police to curb ethnic Serbian militancy in the western republic of Croatia provoked appeals by outraged Serbs for the army to come to their rescue.

The Croatian authorities ordered paramilitary police reinforcements, equipped with automatic weapons and backed by armored cars, into Serbian-inhabited areas to head off a threatened armed insurrection among the republic's 500,000-member Serbian minority.

In neighboring Serbia itself, there were calls by opposition parties for federal armed forces to be sent in to protect the Serbs in Croatia. If not, "we will send our own armed volunteer units to defend our people and our fatherland," declared Vuk Draskovic, leader of the extreme nationalist Serbian Renaissance Movement.

Many Serbs fear they will become second-class citizens in Croatia if its new nationalist government, which won a landslide majority in spring elections, secedes as threatened from the Yugoslav federation. Croatia is one of the most prosperous of the federation's six republics, and its government -- along with that of neighboring Slovenia -- is demanding that the federation be replaced by a much looser confederation of sovereign states.

Yugoslav federal President Borisav Jovic cut short his visit to the United Nations in New York and flew home to deal with the worsening situation at a crisis meeting of Yugoslavia's eight-man executive body, set for Tuesday in Belgrade.

Several hundred ethnic Serbs have been arrested in Croatia since Saturday, after days of clashes between Serbian demonstrators and Croatian riot police in the towns of Petrinja, Glina and Dvor, 40 miles south of Zagreb, the Croatian capital.

The police were sent in to recover weapons allegedly stolen from local police stations. Serbs allege that Croatian authorities are attempting to disarm their communities in a campaign of anti-Serbian "state terror."

Farther south, ethnic Serb authorities in the town of Knin -- the nerve center of Serbian political activism in Croatia -- proclaimed regional autonomy from Zagreb, basing their declaration on an unofficial August referendum in which 99 percent of Serbs in Croatia opted for limited self-rule. Croatian authorities had outlawed the referendum yet allowed it to take place.

Today, Serbian vigilantes armed with shotguns and pistols dragged tree stumps across roads and dug up asphalt on main routes around Knin to the Adriatic coast to hamper police movements, and a Knin radio station broadcast calls on Serbs to resist police incursions by force.

As reports of today's events filtered through to Belgrade, capital of both the Serbian republic and the Yugoslav federation, the city was seized by a mood of anger verging on hysteria. At a news conference called by Serbian opposition parties to discuss a proposed boycott of Dec. 9 elections -- just announced by the ruling Serbian Socialist (formerly Communist) Party -- irate politicians denounced the plight of their fellow Serbs in Croatia.

Party leaders warned of the threatened return to Croatia of the "dark Ustashe terror," a reference to the 1941-1945 civil war in which hundreds of thousands of Serbian civilians died in concentration camps run by the pro-fascist Independent State of Croatia.

President Jovic flies back tonight into an almost impossible dilemma. As a Serb and a longstanding associate of Serbia's forceful president, Slobodan Milosevic, Jovic will be under intense presssure to side with his countrymen in Croatia. Yet any such move is certain to incense Croatian leaders, who have long accused Serbia of backing the Serbian militants in Croatia.