BELGRADE, OCT. 2 -- Yugoslavia's federal leadership set itself tonight on a collision course with the western republics of Croatia and Slovenia.

At a marathon emergency session in Belgrade, the eight-man Federal Presidency demanded the withdrawal of Croatian paramilitary police from areas of the republic where they have been confronting armed Serbian separatists over the past week.

The Presidency also overruled moves by Slovenia last week to bring defense forces on its territory under Slovenian control. Both moves by the Presidency are likely to be strongly opposed by the independent-minded leaderships of Croatia and Slovenia and may precipitate an open split in the already divided federation.

In its statement, the Presidency demanded the release of people "unjustly" detained by Croatian police, who in the past week have arrested more than 200 Serbs in search operations for weapons taken from police stations.

But the Presidency also called on the Serbs to hand in their weapons and dismantle barricades they have set up to block main roads and hamper police movements.

The Croatian Interior Ministry had already announced withdrawal of most police units, but the announcement made no mention of the situation around the southern town of Knin, where Serbian vigilantes remained in control tonight and where thousands of Serbian demonstrators are demanding that the federal army be sent in. The Presidency avoided making that decision tonight.

In contrast, the Presidency took a tough stand against Slovenia. It declared null and void constitutional amendments passed by the Slovenian parliament on Friday giving Slovenia control of defense forces in the republic. In a strong statement, the Presidency said the Defense Ministry in Belgrade remained in control of the 5th Army Region, which includes both Slovenia and Croatia. It said Slovenia's action threatened the constitutional order and integrity of Yugoslavia, something the Presidency "cannot and will not allow."