BELGRADE, DEC. 25 -- Serbia's renamed communists overwhelmingly won the Yugoslav republic's first multi-party election in five decades, taking 194 seats in the 250-seat parliament, according to final official returns from Sunday's second-round balloting.

Serbia, the largest of the six Yugoslav republics, and Montenegro, the smallest -- which also completed voting Sunday -- were the only two to choose communist governments in free elections that began last spring throughout Yugoslavia, setting the stage for heightened political tensions in the ethnically hybrid country. The other four republics voted heavily for anti-communist center-right or nationalist parties.

In Slovenia, the most Westernized Yugoslav republic, 95 percent of the electorate voted Sunday to authorize secession from Yugoslavia unless the country can be reorganized into a loose confederation of sovereign states within six months. Croatia, which lies between Slovenia and Serbia, has supported the Slovenes' position, while Serbia, home to 40 percent of Yugoslavia's 23.5 million inhabitants, strongly opposes it.

Yugoslavia's defense minister, Gen. Veljko Kadijevic, has threatened army intervention to preserve the integrity of the Yugoslav union, while a recent CIA report suggested that the country could fall apart within 18 months because of ethnic tensions and political infighting between republics.