DONETSK, Ukraine — This eastern Ukrainian city took another step toward mob rule Thursday as pro-Russian separatists stormed the state prosecutor’s office and forced dozens of riot police deployed to guard the building into a humiliating surrender. The attackers, who threw stones and wielded sticks, were backed by a crowd of at least 1,000 men and women of all ages. They chanted “fascists” and “traitors” at the riot police and waved Russian flags as well as those of the self-styled Donetsk People’s Republic.