Police formed a cordon round the building in Athens, before going from room to room to evacuate protesters.
Former employees have occupied the building since the government closed ERT and sacked its 2,600 staff in June.
Greece’s conservative-led coalition said the state broadcaster cost too much to run in an economic crisis.
The closure of ERT prompted a left-wing party to withdraw from the governing coalition of PM Antonis Samaras in protest – a move which almost brought down the government.
Following the announcement of ERT’s closure in June, hundreds of staff refused to leave the building and continued to broadcast their programming via the internet.
Early on Thursday, Greek police arrived to secure the building in Agia Paraskevi, a suburb in the north of Athens.
Riot police used tear gas to disperse about 200 protesters outside the building, and then cleared each room inside.
Twenty or so workers were led out but three refused to go and were arrested. Police have now surrounded the building and protesters are gathering.
The state-run Athens News Agency reported that Panagiotis Kalfagiannis, a journalist and head of the ERT employees’ union, Pospert, was one of those held for public order offences.
ERT was Greece’s only TV broadcaster until the advent of private TV channels in 1989.
Despite several major overhauls to keep up with fierce private competition, a fall in ERT’s ratings in the mid-1990s triggered a long-running debate about its cost and efficiency.
In June, as Greece attempted to satisfy international creditors that it was fulfilling its debt restructuring and bailout commitments, PM Antonis Samaras announced the closure of ERT.
Antonis Samaras said the Greek exchequer could no longer afford to pay for a public broadcaster that cost more than $400,000 a year to run, and has refused to reinstate ERT unless it accepts a complete restructuring.
An interim TV station, called Public TV or DT, has been broadcasting in Greece since July while a restructured public broadcaster, called Nerit, is not expected to begin operating before 2014.
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