ZAGREB, YUGOSLAVIA, JAN. 22 -- A midnight deadline set by Yugoslavia's central government for all "illegal paramilitary units" in the country to turn in their weapons or have them confiscated by the army passed quietly last night with no apparent action by troops to enforce the decree.

The ultimatum, forced through the ethnically hybrid country's collective presidency by Serbia, the largest of Yugoslavia's six constituent republics, was aimed chiefly at rapidly growing police and local militia forces in the republics of Slovenia and Crotia.

Newly elected nationalist governments in both those republics, the most prosperous and Westernized in the Yugoslav union, have threatened to declare their independence unless Yugoslavia is transformed into a much looser confederation of sovereign states, a prospect strongly opposed by the army and the Serbian government, both of which are still dominated by old-line communist officials.

As the disarmament deadline passed last night, the state presidency announced in Belgrade, capital of both Yugoslavia and Serbia, that it intended to prosecute civilians who ignored the decree, but it made no mention of arms confiscations by troops, an apparent retreat from its earlier threat. Some observers here had predicted that any Yugoslav army move to seize arms in Croatia, Slovenia and other restive regions of this crisis-torn country would have swiftly brought on civil war.

"What is most important is that there will be no movement or action by the army," Stipe Mesic, Croatia's representative on the eight-member presidency, told reporters tonight. "There is no danger of a military option."

Authorities here in Zagreb, the Croatian capital, appealed for calm as the deadline expired, and local police had assembled in force to defend the city against any army threat.

But the danger lingers, said Croatian presidential spokesman Mario Nobilo. "We are still afraid that conservative {Communist} circles will use the situation in the {Soviet Baltic republics} and the war in the gulf as cover to stage a classic coup."