BELGRADE, OCT. 19 -- President Borisav Jovic today called for an end to ethnic confrontations that in recent weeks have threatened to pitch Yugoslavia's two largest groups, the Serbs and the Croats, into civil war.

"Only through negotiations can we agree to live together," Jovic warned in an emotional speech to the parliament. Any peaceful solution to the conflicts in this country of 23 million people "is preferable to conflict and bloodshed," he said.

But even as he spoke, the western Republic of Croatia was mobilizing reserve police units amid fears that they would be used to put down a campaign of armed defiance by members of the Serbian community around the southeastern Croatian town of Knin.

For the past eight weeks, militant Serbs in Knin have been barricading roads and mining railroad tracks to back their demands for equal treatment and regional autonomy from the Croatian authorities in Zagreb, the republic's capital. Today, the militants rejected an appeal to reopen the road and rail links made by the transportion minister, who traveled from Belgrade to try to ease the situation.

In Yugoslavia today, the present is being driven by the past, and Croatia's 500,000 Serbs have long and bitter memories of the slaughter of uncounted thousands of Serbs in World War II concentration camps run by the fascist puppet "Independent Capital State of Croatia."

Suppressed for 40 years by Yugoslavia's charismatic postwar leader, Tito, these memories are now, 10 years after his death, firing rival nationalist passions across this patchwork state of six republics and two autonomous provinces.

In recent weeks, Serbs have clashed with Croats and Moslems. In prosperous Slovenia, federal military police stormed into a military building in the capital, Ljubljana, in an unsuccessful attempt to prevent the takeover of local defense forces by the Slovenian authorities.

The defeat of the ruling Communists in spring elections in Slovenia and Croatia brought new nationalistic governments to power determined to confront what they see as the expansionist ambitions of Serbia, Yugoslavia's largest republic.

"We are threatened by a totalitarian gang of Serbian Communists who will stop at nothing to cling onto power," said Zarko Domljan, new speaker of the coalition-led parliament.

To Domljan and other Croatian leaders, the Serbian rebellion around Knin is a plot hatched by Serbia's ruling Socialist (formerly Communist) Party to bring down the new government in Zagreb.

Last week in Zagreb, a crowd of 15,000 Croatian nationalists shouted "Give us guns" and "We're marching against Knin."

Riding a tide of patriotic frenzy, Croatian President Franjo Tudjman on Tuesday won roars of approval from a crowd of up to a half-million Croats in downtown Zagreb with the pledge: "If needed, the entire Croatian nation will fight to preserve the democracy it won this spring."

Two weeks ago, Croatia and Slovenia published detailed proposals for a Yugoslav confederation as a way out of the crisis. But their demands for a loose alliance of independent states -- each with its own government, army and foreign policy -- only sharpened the battle lines at the federal level.

The confederation proposals were denounced in Serbia as seeking the breakup of Yugoslavia. They stiffened the resistance of the Serbs in Croatia who fear confederation will bring back the nightmares of the 1940s.

The deputy mayor of Knin and leading spokesman of the Serbs, Lazar Macura, has accused Croatian authorities of provoking the Serbs by attempting to disarm reserve police in Serbian areas.

"They are behaving like Ustashe," he said, in a reference to the black-shirted Croatian fascists who first disarmed and then led the Serbian civilians to wartime death camps. "If they try to storm our barracades, this time we will use our guns against them."

The crisis has been worsened by the paralysis now gripping the top federal bodies. The eight-man federal presidency that succeeded Tito is deadlocked between members from rival republics. Jovic, the current state president, is a Serb accused of trying to stamp a centralized pro-Serbian model of federation on the rest of the country.

In his speech to the parliament today, Jovic tried to soothe these fears by saying the two contrasting visions of Yugoslavia -- federal and confederal -- should be considered. "We have not yet burned our last bridges," he told legislators.

However, most leaders in the republics want to delay any final decision until after multi-party elections due to be held in the remaining four republics before the end of the year.

Many Yugoslavs want the international community to step in now to prevent war. "We are like a family that is being terrorized by a delinquent son-Serbia," said Hide Biscevic, editor of the Croatian daily Vjesnik.

"Europe and the international community, which created Yugoslavia in 1918, have locked us in this house. Now it's time they let us out."