"We have been preparing this for more than a month," a rebel named Oleksandr Matyushyn told AFP as five separatist gunmen stood guard at its main entrance and bank staff filed out of the building.
"We want the tax revenues to stay here instead of going to Kiev," he added.
Matyushyn said his unit had entered discussion with local administrators about transferring control over the local treasury and tax collection service to the separatist leader of self-proclaimed "Donetsk People's Republic".
Donetsk and the neighboring heavily Russified region of Lugansk declared independence from Kiev in disputed May 11 referendums whose legitimacy was rejected by Kiev and decried as a sham by the West.
Russian President Vladimir Putin urged Ukraine's new pro-Western leaders to listen closely to the opinions expressed in the referendums but also refused to recognised the two regions' independence from Kiev rule.
Federal forces' two-month campaign to regain control over the industrial region of seven million people has now claimed the lives of more than 370 civilians and fighters on both sides.