Complex sentence: At the same time, the Bjelovar garrison was blockaded and its utilities and supplies were cut, while negotiations on surrender of the garrison were started involving the civilian crisis headquarters, presided over by Jure Šimić.
Split into simpler sentences: At the same time, the Bjelovar garrison had been blockaded, and its utilities and supplies were cut. Negotiations ensued for the surrender of the garrison, led by the civilian crisis headquarters presided over by Jure Šimić.