KNIN, YUGOSLAVIA, AUG. 18 -- Serbs with shotguns and hunting rifles sealed roads into southwestern Croatia today in what the republic's presidency said amounted to an armed insurrection.

The Serbs, who account for 11 percent of Croatia's 5 million people, planned a referendum Sunday on their autonomy within the republic and set up roadblocks to keep police from interfering. The Croatian government has declared the vote illegal.

A statement by the Croatian presidency, reported on Croatian radio, said tensions had "assumed the form of an armed insurrection." Both sides appeared to be trying to avoid an armed conflict, however. Police did not try to move into the region.

Radio Knin reported earlier, after a conversation with local Serbian leader Milan Babic, that contacts were being made with Croatian President Franjo Tudjman and the head of Croatian police to defuse the dispute.

Babic appealed over local radio to Serbs manning roadblocks to let traffic pass.

"If the police try to break through in their armored cars, we will fry them like chickens in a pan," said a Serb armed with a double-barrel shotgun and standing guard at a roadblock nine miles north of Knin. He refused to identify himself.

Other armed Serbs felled trees and dumped rocks on the main highway south of the Croatian capital, Zagreb, to seal off the area. In a village near Knin, some Serbs were seen armed with light machine guns.

The roadblocks sealed off resorts on the Adriatic coast and left carloads of German and Italian tourists stuck facing armed men.

The increasingly bitter disagreements between Serbs and Croats, Yugoslavia's two largest ethnic groups, threaten to tear the fragile federation apart.

Serbs have accused Tudjman's new, reform-minded government of fostering nationalism. His administration advocates turning the country into a loose federation of six nearly independent states.

Serbia's hard-line Communist president, Slobodan Milosevic, and Yugoslavia's Communist-dominated armed forces oppose this and favor greater centralization and federal control. On Friday, Yugoslav air force fighters intercepted a flight of Croatian police helicopters before they could reach this region near the Adriatic coast.

Official Croatian radio today accused the Serbian minority of trying to secede and join the Serbian republic. Croatian officials fear that would foster domination of Yugoslavia by Serbia.