Patriarch Dimitrios I, 76, spiritual leader of the world's 300 million Eastern Orthodox Christians and a champion of Christian unity, died yesterday of a heart attack in Istanbul, Turkey. Dimitrios held the title Archbishop of Constantinople and was primus inter pares, first among equals, of the five senior Eastern Christian leaders. He presided over worldwide Orthodoxy, which divided into 14 churches of many ethnic groups. Six million followers of Eastern Orthodoxy live in the United States. The patriarch traveled widely abroad to promote unity among the Christian churches.