Place names that are of Slavic origin abound in Romania, indicating that a significant Slavic-speaking population used to live in the territory. The first Slavic groups settled in Moldavia and Wallachia in the 6th century, in Transylvania around 600. After the Avar Khaganate collapsed in the 790s, Bulgaria became the dominant power of the region, occupying lands as far as the river Tisa. The Council of Preslav declared Old Church Slavonic the language of liturgy in the First Bulgarian Tsardom in 893. The Romanians also adopted Old Church Slavonic as their liturgical language.