FIERCE fighting between ethnic Croats and Serbs plunged Yugoslavia into its worst day of violence this year.Witnesses near the town of Glina, 30 miles south of Croatia's capital, Zagreb, described scenes of carnage. A local official in the nearby town of Dvor na Uni said about 100 Croatian national guardsmen and police had been killed since fighting erupted Friday morning. The Army, ostensibly a buffer between warring Roman Catholic Croats and Christian Orthodox Serbs, has thrown its battle tanks into the fighting since Thursday, when it killed nine national guardsmen in eastern Croatia. So far the Serbian-dominated Army has targeted mainly Croatian national guard and police positions. That has prompted charges from Croatia that in defending the 600,000-strong Serbian minority in the republic, the Army may also be laying out the boundaries of a "Greater Serbia" should Yugoslavia break up into independent states. Prime Minister Ante Markovic and representatives of the collective state presidency fly to Brussels today for talks with the European Community.