The aim of this study was to evaluate the influence of selected processing conditions of beetroot spontaneous fermentation, including slice thickness (2, 4, and 6 mm), temperature (15, 20, and 25^oC) and fermentation time (1-15 days), on the physicochemical properties of the resulting leaven (total soluble solid content, turbidity, pH, titratable acidity, color parameters, and content of total carbohydrates, total phenolics, total betacyanins and total betaxanthins). All tested conditions had a significant impact on the properties of beet leaven. Among them, fermentation time and temperature were mostly decisive. Slice thickness was important only for short-time fermentation. The preferable pH below 4.1 was achieved between the 4^th and 6^th day of fermentation, depending on process temperature. Titratable acidity steadily increased and pH decreased throughout the fermentation process, with more pronounced changes observed at the highest temperature. The highest values of total soluble solids (7.25%) and turbidity (1,100 NTU) were noted on the 13^th day of fermentation of the thinnest slices at lower temperatures (15^oC and 20^oC, respectively). The color of the leaven darkened with increasing fermentation time, but the changes in the b* parameter were the most notable. Changes in the content of bioactive compounds were dynamic during beetroot spontaneous fermentation, but higher temperatures promoted increased total phenolic content and total betaxanthin content in the leaven. The optimal fermentation conditions in terms of all tested leaven properties were determined at 6 mm, 20^oC and 7 days for slice thickness, temperature and processing time, respectively.