Russian-speaking regions of Ukraine spun further out of the central government’s control on Sunday as a mob stormed a police station in this Black Sea port and freed from detention 67 pro-Russian militants, on the same day that Ukraine’s prime minister was visiting the city. It was intended to be a chance for the prime minister, Arseniy P. Yatsenyuk, to express condolences for the dozens of people who died here on Friday in street fighting and in a horrific fire at a trade union building, and to reinforce the government’s narrative that Russia and inept or disloyal local police were to blame.