NOVSKA, Yugoslavia (AP) _ Armed rebel Serbs kept Europe's main road link with Turkey and the Middle East shut for a second day Thursday in their battle to cut Croats in eastern Croatia off from the rest of the republic.

The Zagreb-Belgrade highway, normally flooded with traffic, has been virtually deserted this wartorn summer.

On Thursday, backed-up trucks idled at the side of the road. Croatian police started turning motorists back from the dangerous route just 13 miles southeast of Zagreb.

At Novska, 50 miles outside the Croatian capital, a Croat policeman warned against traveling on. Serb guerrillas had control of a 2 1/2 -mile stretch further down the road, he said.

If the Serbs control the highway, the Croats are left with virtually no way of supplying the east Croatian region of Slavonia, where Serb irregulars appear poised for a big battle to capture Osijek, Slavonia's Croat-dominated capital and the fourth largest city in the republic.

Rural back roads are still passable, but the roadblocks thrown up by Serbs and Croats months ago at best delay and at worst prevent movement on them.

At the last lone outpost of four Croats on the highway, Zeljko Babic gazed through binoculars at several vehicles and a crowd of people on the next, distant overpass.

''Those are Martic's men,'' said Babic, referring to Milan Martic, self- styled commander of Serb guerrillas based in the Serb enclave of Krajina in western Croatia.

With the exception of four hours' uneasy sleep, Babic said he had been at his post since shooting started before dawn Wednesday, when Serb rebels reportedly blocked the highway by commandeering trucks and buses.

Croatian highway workers managed to tow all but three of the trucks away Wednesday during a midday firefight between Serbs and Croats, according to highway worker Ivan Turek.

The federal army, accused by Croatia of supplying the Serbs with weapons and backing their attacks, did not appear to have intervened directly in the highway standoff by noon Thursday.

But Babic said he had seen two tanks and five armored vehicles in nearby villages.

The army, well-equipped and led by a Serb-dominated officer corps, strongly denies aiding the Serbs, saying it acts only to separate the warring sides or fires when attacked.

Babic believes local commanders are now acting largely independently. ''The army is not obeying its high command,'' he said.

His 16-year-old companion, Ranko Pribetic, wearing a shiny new badge of the Croatian security forces, brandished an automatic rifle.

''I am not afraid to die,'' he said. Pointing at the distant overpass and its alleged band of Serbs, he added, ''I have friends up there on that bridge, even old schoolmates. What's there to think, all I know is that I have to fight.''