ZAGREB, Yugoslavia (AP) _ A cease-fire was observed in most of secessionist Croatia today, but Croatian defense officials said shelling continued near the eastern city of Vukovar and the Adriatic resort of Dubrovnik.

The Yugoslav news agency Tanjug reported artillery duels in Vukovar, a town on the republic's border with rival Serbia that has been under siege for two months. The report said about 250 shells fell on the city.

Tanjug quoted Croatian radio as saying the defenders ''lacked everything'' and that food supplies were running out.

Explosions were also heard this morning in Zupa Dubrovacka, a resort area south of Dubrovnik, Zagreb radio reported. It said artillery fire toward Zupa Dubrovacka began in early morning, and police blocked roads in the area.

Defense officials in Zagreb, speaking anonymously, confirmed fighting took place near Vukovar and Dubrovnik, but gave no further details.

The cease-fire was mediated by the European Community, took effect Saturday, and is the 10th truce of the civil war.

The Serbian-dominated army and ethnic Serb militias have seized about a third of Croatia's territory in fighting that has claimed more than 1,000 lives since Croatia declared independence June 25.

In the Croatian capital, 1,000 people thronged the central square today to support 150 Zagreb University students from Dubrovnik on the first day of a hunger strike. The students are protesting what they see as international indifference to their medieval Adriatic town, which has been besieged by the federal army and Serbian and Montenegran irregulars for 22 days.

Waving placards reading ''Europe, you are guilty,'' and ''Visit Serbia before Serbia visits you,'' the students called on world politicians to press for an end to the land and sea blockade of Dubrovnik.

Earlier, the Dutch Defense Ministry announced the EC would withraw a squadron of helicopters used to transport its cease-fire observers in Yugoslavia because of the increasingly dangerous situation.

The three Dutch and three Italian helicopters, which have come under fire several times, will begin returning to their home bases on Wednesday, ministry spokesman Wiebe Alkema said.

The helicopters have been used to move the 200 or so unarmed observers sent to monitor cease-fire violations as part of the EC's peace efforts. They also have been used to transport wounded.