"In the office, exercising the responsibilities entrusted to us by the people of Catalonia," said Josep Rull, who until last week was the region's senior official in charge of territorial affairs.
Two police officers entered and left the building, followed minutes later by Mr Rull, who told reporters and supporters that he would continue carrying out his agenda.
Spanish authorities say deposed officials will be allowed to take their personal belongings from official buildings, but are barred from performing any official duties.